refactor(timeline): extract shared EventHeader for pill + event-card

EventCluster's same-year header re-implemented EventPill's glyph circle,
serif title, provenance subtitle, and the curator edit anchor near-
verbatim — the third copy of that markup. They now share a single
EventHeader component (glyph via GlyphLabel, title, `{date} · provenance`
subtitle, optional sr-only letter count, and the canEditEvent-gated edit
pencil); EventPill keeps only its pill border, EventCluster only its card
chrome. Second half of review finding #5 (Architect-1). No behavior
change — EventPill/EventCluster/YearBand/TimelineView specs stay green.

Refs #850
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.specify/rtm.md

@@ -194,3 +194,18 @@
 | REQ-009 | TimelineView threads canWrite through the year-band path and the undated bucket (identical gate) | #842 | timeline-curator-affordances | `frontend/src/lib/timeline/TimelineView.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `TimelineView.svelte.spec.ts#threads canWrite to a curated event in both a year band and the undated bucket`, `#threads canWrite to a curated HISTORICAL world band in both paths` | Done |
 | REQ-010 | person add-event opens #781 create form prefilled with the person, returns to /persons/{id} on save | #842 | timeline-curator-affordances | `frontend/src/routes/persons/[id]/PersonCard.svelte`, `frontend/src/routes/zeitstrahl/events/new/+page.server.ts` (#781) | `PersonCard.svelte.spec.ts#shows an add-event link pre-seeded with the person to a curator` (URL), `zeitstrahl/events/new/page.server.spec.ts#redirects to /persons/{id} when originPersonId is a valid UUID` (#781) | Done |
 | REQ-011 | non-curator direct nav to /zeitstrahl/events/new or /{id}/edit → 403 (existing #781 route guard, regression) | #842 | timeline-curator-affordances | `frontend/src/routes/zeitstrahl/events/new/+page.server.ts`, `frontend/src/routes/zeitstrahl/events/[id]/edit/+page.server.ts` (#781, unchanged — both share `requireWriteAll`) | `zeitstrahl/events/new/page.server.spec.ts#throws 403 for an authenticated user without WRITE_ALL`, `zeitstrahl/events/[id]/edit/page.server.spec.ts#throws 403 for a user without WRITE_ALL` | Done |
+| REQ-001 | `/zeitstrahl` renders a single chronological timeline with no grouping-mode control (no toggle/Thema/drawer) | #850 | inline-event-clustering | `frontend/src/routes/zeitstrahl/+page.svelte`, `frontend/src/lib/timeline/TimelineView.svelte` | `page.svelte.spec.ts#renders the meta sub-line with range and counts, no grouping segment`; absence of `GroupingControl`/`LetterBucket`/`BucketHeaderChip` (never ported) | Done |
+| REQ-002 | curated event with ≥1 same-year linked letter → contained card (event header + glyph/title/date·provenance/edit + compact letter cards) | #850 | inline-event-clustering | `frontend/src/lib/timeline/EventCluster.svelte`, `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/eventClustering.ts` | `EventCluster.svelte.spec.ts#renders a data-testid event-card with the event title once`, `#shows the event-edit link for a curator`, `#renders its letters as compact a.lcard.ev cards`; `YearBand.svelte.spec.ts#renders a curated event with a same-year linked letter as one event-card, title once, no separate pill` | Done |
+| REQ-003 | event card body > 5 letters → first 5 + keyboard-operable show-more/less toggle (aria-expanded, ≥44px) | #850 | inline-event-clustering | `frontend/src/lib/timeline/EventCluster.svelte`, `frontend/src/lib/timeline/eventClustering.ts` (`CLUSTER_PREVIEW=5`) | `EventCluster.svelte.spec.ts#shows the first 5 of 8 letters + a show-more toggle that expands to 8 then back to 5`, `#renders no show-more toggle when the cluster holds 5 or fewer letters` | Done |
+| REQ-004 | linked letters in a year other than the event's band → labeled ✉ text-header card (no pill, no edit link) per such year | #850 | inline-event-clustering | `frontend/src/lib/timeline/EventCluster.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `EventCluster.svelte.spec.ts#renders a cross-year text header (✉ title, no event-edit, no pill) when no event is given`; `YearBand.svelte.spec.ts#renders a cross-year cluster (event not in this band) as a ✉ text-header card holding it` | Done |
+| REQ-005 | event/derived/world-band with no linked letters → existing plain pill/world-band (unchanged) | #850 | inline-event-clustering | `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/EventPill.svelte`, `frontend/src/lib/timeline/WorldBand.svelte` | `YearBand.svelte.spec.ts#renders a curated event with NO linked letters as a plain EventPill, no card` | Done |
+| REQ-006 | letter linked to no curated event → loose chronological letter (alternating; density strip past 12) | #850 | inline-event-clustering | `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/eventClustering.ts` | `eventClustering.spec.ts#keeps a letter with no linkedEventId loose`; `YearBand.svelte.spec.ts#renders loose (unlinked) letters in a sparse year as alternating .letter-row, no card` | Done |
+| REQ-007 | loose-letter layout + density strip count ONLY non-event-linked letters; clustered letter never also loose | #850 | inline-event-clustering | `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/eventClustering.ts` | `eventClustering.spec.ts#places each letter in exactly one place`; `YearBand.svelte.spec.ts#counts only loose letters in the density strip; event letters stay in the card` | Done |
+| REQ-008 | letter whose only linking event is filtered out (absent from lookup) → loose, never re-introduces the event (filter-then-cluster) | #850 | inline-event-clustering | `frontend/src/lib/timeline/eventClustering.ts` (`splitYearLetters` filters via `eventLookup`) | `eventClustering.spec.ts#keeps a letter whose linkedEventId is absent from the lookup loose (filter-then-cluster, REQ-008)` | Done |
+| REQ-009 | `TimelineEntryDTO` carries nullable `linkedEventId` for LETTER entries, one batched pass, no new column/migration, not @Schema(REQUIRED); a multi-event letter links deterministically (earliest date, then id) | #850 | inline-event-clustering | `backend/.../timeline/TimelineEntryDTO.java`, `backend/.../timeline/TimelineService.java#resolveLetterEventLinks`, `frontend/src/lib/generated/api.ts` | `TimelineServiceTest#letter_in_a_curated_events_documents_carries_that_events_id`, `#letter_in_no_curated_event_has_null_linkedEventId`, `#multi_event_letter_links_deterministically_to_the_earliest_event` | Done |
+| REQ-010 | event/letter/sender/receiver text via Svelte `{...}` escaping; no `{@html}` anywhere in `lib/timeline/` (grep gate, CWE-79) | #850 | inline-event-clustering | `frontend/src/lib/timeline/*.svelte` | `timeline-no-raw-html.spec.ts#no timeline component contains the raw-HTML directive`; `EventCluster.svelte.spec.ts#renders an HTML-bearing event title verbatim as text, never as markup` | Done |
+| REQ-011 | wrapping header keeps #842 add-event CTA + #780 filter trigger; meta-line drops the grouping segment | #850 | inline-event-clustering | `frontend/src/routes/zeitstrahl/+page.svelte` | `page.svelte.spec.ts#renders the meta sub-line with range and counts, no grouping segment`, `#drops the letters segment instead of showing "0 Briefe"` (no "Gruppierung") | Done |
+| REQ-012 | show-more/less labels are new Paraglide keys in de/en/es; unused #827 grouping/Thema keys removed | #850 | inline-event-clustering | `frontend/messages/{de,en,es}.json`, `frontend/src/lib/timeline/EventCluster.svelte` | `messages.spec.ts#zeitstrahl visual-fidelity keys are present in all locales` (now asserts `timeline_bucket_show_more`/`_less`; `timeline_grouping_date` removed) | Done |
+| REQ-013 | `GET /api/timeline` failure → existing localized error state via `getErrorMessage(code)` (unchanged #779) | #850 | inline-event-clustering | `frontend/src/routes/zeitstrahl/+page.svelte` (unchanged error path) | covered by #779 `zeitstrahl` error-state tests (regression — no change) | Done |
+| REQ-014 | HISTORICAL curated event with ≥1 linked letter keeps its full-width WorldBand — never clusters into a card (preserves #779 REQ-009); its letters stay loose | #850 | inline-event-clustering | `frontend/src/lib/timeline/eventClustering.ts` (`buildEventLookup` excludes HISTORICAL) | `eventClustering.spec.ts#excludes a HISTORICAL event so its letters stay loose, keeping its WorldBand (REQ-014)`; `TimelineView.svelte.spec.ts#keeps a HISTORICAL event a WorldBand with a same-year linked letter — never clusters (REQ-014)` | Done |
+| REQ-015 | a cross-year ✉ card is placed at its earliest linked letter's chronological position in the band — never appended after later-dated loose letters | #850 | inline-event-clustering | `frontend/src/lib/timeline/YearBand.svelte` | `YearBand.svelte.spec.ts#interleaves a cross-year card before a later-dated loose letter in the same band (REQ-015)` | Done |

backend/src/main/java/org/raddatz/familienarchiv/timeline/TimelineEntryDTO.java

@@ -28,6 +28,13 @@ import java.util.UUID;
  * They are deliberately NOT {@code @Schema(requiredMode = REQUIRED)} so the generated TypeScript
  * types stay optional.
  *
+ * <p><b>Letter→event link ({@code linkedEventId}):</b> for a {@link Kind#LETTER} entry, the id of
+ * the curated {@link TimelineEvent} whose {@code documents} set contains the letter's document, or
+ * {@code null} when the letter is referenced by no curated event (#850). Computed on read from the
+ * existing {@code timeline_event_documents} join — no new column. {@code null} for non-letter
+ * entries. Deliberately NOT {@code @Schema(requiredMode = REQUIRED)} so the generated TypeScript
+ * type stays optional.
+ *
  * <p>Callers of {@code TimelineEventService.assembleDerivedEvents()} must independently enforce
  * {@code READ_ALL} authorization before invoking that method (see ADR-043).
  */
@@ -47,6 +54,7 @@ public record TimelineEntryDTO(
         DerivedEventType derivedType,
         UUID rootTagId,
         String rootTagName,
-        String rootTagColor
+        String rootTagColor,
+        UUID linkedEventId
 ) {
 }

backend/src/main/java/org/raddatz/familienarchiv/timeline/TimelineEventService.java

@@ -267,7 +267,7 @@ public class TimelineEventService {
                         p.getBirthDate(), null,
                         p.getDisplayName(), EventType.PERSONAL,
                         null, null, List.of(p.getId()), DerivedEventType.BIRTH,
-                        null, null, null))
+                        null, null, null, null))
                 .toList();
     }
 
@@ -279,7 +279,7 @@ public class TimelineEventService {
                         p.getDeathDate(), null,
                         p.getDisplayName(), EventType.PERSONAL,
                         null, null, List.of(p.getId()), DerivedEventType.DEATH,
-                        null, null, null))
+                        null, null, null, null))
                 .toList();
     }
 
@@ -304,7 +304,7 @@ public class TimelineEventService {
                         null, null,
                         List.of(r.getPerson().getId(), r.getRelatedPerson().getId()),
                         DerivedEventType.MARRIAGE,
-                        null, null, null));
+                        null, null, null, null));
             }
         }
         return result;

backend/src/main/java/org/raddatz/familienarchiv/timeline/TimelineService.java

@@ -80,13 +80,20 @@ public class TimelineService {
         // Resolve generation person IDs once — used across all three layers
         Set<UUID> genPersonIds = resolveGenerationPersonIds(filter.generation());
 
+        // Fetch curated events once; the events that survive the filter below feed both the
+        // event entries and the batched letter→event link pass (resolveLetterEventLinks), so the
+        // membership pass costs no extra query and touches only on-screen events. REQ-009.
+        List<TimelineEvent> allEvents = eventRepository.findAll();
+
         // ── curated events ───────────────────────────────────────────────────
         List<TimelineEntryDTO> entries = new ArrayList<>();
-        for (TimelineEvent ev : eventRepository.findAll()) {
+        List<TimelineEvent> filteredEvents = new ArrayList<>();
+        for (TimelineEvent ev : allEvents) {
             if (!passesTypeFilter(ev.getType(), filter.type())) continue;
             if (!passesPersonFilter(ev.getPersons(), filter.personId())) continue;
             if (!passesGenerationFilter(ev.getPersons(), genPersonIds)) continue;
             if (!passesYearFilter(ev.getEventDate(), ev.getPrecision(), filter)) continue;
+            filteredEvents.add(ev);
             entries.add(mapEvent(ev));
         }
 
@@ -107,8 +114,9 @@ public class TimelineService {
             letters.add(doc);
         }
         Map<UUID, RootTag> rootByDocId = resolveLetterRootTags(letters);
+        Map<UUID, UUID> eventByDocId = resolveLetterEventLinks(letters, filteredEvents);
         for (Document doc : letters) {
-            entries.add(mapDocument(doc, rootByDocId));
+            entries.add(mapDocument(doc, rootByDocId, eventByDocId));
         }
 
         return bucket(entries);
@@ -229,11 +237,13 @@ public class TimelineService {
                 null,
                 null,
                 null,
+                null,
                 null
         );
     }
 
-    private TimelineEntryDTO mapDocument(Document doc, Map<UUID, RootTag> rootByDocId) {
+    private TimelineEntryDTO mapDocument(Document doc, Map<UUID, RootTag> rootByDocId,
+                                         Map<UUID, UUID> eventByDocId) {
         RootTag root = rootByDocId.get(doc.getId());
         return new TimelineEntryDTO(
                 Kind.LETTER,
@@ -251,10 +261,50 @@ public class TimelineService {
                 null,
                 root == null ? null : root.id(),
                 root == null ? null : root.name(),
-                root == null ? null : root.color()
+                root == null ? null : root.color(),
+                eventByDocId.get(doc.getId())
         );
     }
 
+    /**
+     * Resolves each letter's linked curated event in one batched pass, keyed by document id: the
+     * event whose {@code documents} set contains the letter (REQ-009). A single doc→event map is
+     * built over the already-loaded events — no per-letter query ({@code TimelineEvent.documents}
+     * carries {@code @BatchSize(50)}). When a document is referenced by more than one curated
+     * event, the earliest-dated event wins (then lowest {@code eventId}); the pass runs over a
+     * stably-ordered copy so the link is a deterministic property of the data, not a coin-flip on
+     * the undefined repository iteration order ({@code putIfAbsent} keeps the first = winner). The
+     * map is built only over the events that survived the timeline filter, so the lazy
+     * {@code documents} collection is hydrated for on-screen events alone (finding #10); a letter
+     * whose only linking event was filtered out links to nothing, matching the frontend's
+     * filter-then-cluster (#850). Logs nothing — the assembly path keeps UUIDs out of logs (§2.7).
+     */
+    private Map<UUID, UUID> resolveLetterEventLinks(List<Document> letters, List<TimelineEvent> events) {
+        Set<UUID> letterDocIds = letters.stream().map(Document::getId).collect(Collectors.toSet());
+        if (letterDocIds.isEmpty()) return Map.of();
+
+        // Stable order so a multi-event letter links deterministically: earliest event date
+        // (undated last), then lowest id. putIfAbsent below then keeps the winner (REQ-009).
+        List<TimelineEvent> ordered = events.stream()
+                .sorted(Comparator
+                        .comparing(TimelineEvent::getEventDate,
+                                Comparator.nullsLast(Comparator.naturalOrder()))
+                        .thenComparing(TimelineEvent::getId))
+                .toList();
+
+        Map<UUID, UUID> eventByDocId = new HashMap<>();
+        for (TimelineEvent ev : ordered) {
+            Set<Document> linkedDocs = ev.getDocuments();
+            if (linkedDocs == null) continue;
+            for (Document linked : linkedDocs) {
+                if (letterDocIds.contains(linked.getId())) {
+                    eventByDocId.putIfAbsent(linked.getId(), ev.getId());
+                }
+            }
+        }
+        return eventByDocId;
+    }
+
     /**
      * Resolves each letter's primary root tag in one batched pass, keyed by document id — no
      * per-letter N+1: each letter contributes only its alphabetically-first assigned tag (#835),

backend/src/test/java/org/raddatz/familienarchiv/timeline/TimelineServiceTest.java

@@ -69,10 +69,10 @@ class TimelineServiceTest {
         UUID id2 = UUID.fromString("00000000-0000-0000-0000-000000000002");
         var e1 = new TimelineEntryDTO(Kind.LETTER, DatePrecision.DAY, false, "", "",
                 LocalDate.of(1914, 7, 28), null, "Same Title", null, null, id1, List.of(), null,
-                null, null, null);
+                null, null, null, null);
         var e2 = new TimelineEntryDTO(Kind.LETTER, DatePrecision.DAY, false, "", "",
                 LocalDate.of(1914, 7, 28), null, "Same Title", null, null, id2, List.of(), null,
-                null, null, null);
+                null, null, null, null);
 
         var sorted = List.of(e2, e1).stream()
                 .sorted(TimelineService.WITHIN_BAND_ORDER)
@@ -511,6 +511,106 @@ class TimelineServiceTest {
         verify(tagService, times(1)).resolveRootTags(anyList());
     }
 
+    // ─── letter→event link (#850, REQ-009) ───────────────────────────────────
+
+    @Test
+    void letter_in_a_curated_events_documents_carries_that_events_id() {
+        // REQ-009: linkedEventId = the curated event whose documents set contains the letter.
+        Document letterDoc = docWithDate(LocalDate.of(1915, 5, 1), DatePrecision.MONTH);
+        UUID eventId = UUID.randomUUID();
+        TimelineEvent event = TimelineEvent.builder().id(eventId)
+                .title("Briefe von der Front").type(EventType.PERSONAL)
+                .documents(new HashSet<>(Set.of(letterDoc)))
+                .build(); // no eventDate → event lands undated, leaving the year band to the letter
+        when(eventRepository.findAll()).thenReturn(List.of(event));
+        when(timelineEventService.assembleDerivedEvents()).thenReturn(List.of());
+        when(documentService.getAllForTimeline()).thenReturn(List.of(letterDoc));
+
+        TimelineEntryDTO entry = onlyLetterEntry(timelineService.assemble(noFilters()));
+
+        assertThat(entry.linkedEventId()).isEqualTo(eventId);
+    }
+
+    @Test
+    void letter_in_no_curated_event_has_null_linkedEventId() {
+        // REQ-009: a letter referenced by no curated event → linkedEventId null; the frontend
+        // then renders it as a loose chronological letter (REQ-006).
+        Document letterDoc = docWithDate(LocalDate.of(1915, 5, 1), DatePrecision.MONTH);
+        TimelineEvent event = TimelineEvent.builder().id(UUID.randomUUID())
+                .title("Anderes Ereignis").type(EventType.PERSONAL)
+                .documents(new HashSet<>(Set.of(Document.builder().id(UUID.randomUUID()).build())))
+                .build();
+        when(eventRepository.findAll()).thenReturn(List.of(event));
+        when(timelineEventService.assembleDerivedEvents()).thenReturn(List.of());
+        when(documentService.getAllForTimeline()).thenReturn(List.of(letterDoc));
+
+        TimelineEntryDTO entry = onlyLetterEntry(timelineService.assemble(noFilters()));
+
+        assertThat(entry.linkedEventId()).isNull();
+    }
+
+    @Test
+    void multi_event_letter_links_deterministically_to_the_earliest_event() {
+        // REQ-009: a document referenced by >1 curated event links to the earliest-dated event
+        // (then lowest id), independent of repository iteration order — not a coin-flip on
+        // findAll()'s undefined order.
+        Document shared = docWithDate(LocalDate.of(1916, 5, 1), DatePrecision.MONTH);
+        TimelineEvent earlier = TimelineEvent.builder()
+                .id(UUID.fromString("00000000-0000-0000-0000-000000000001"))
+                .title("Frühes Ereignis").type(EventType.PERSONAL)
+                .eventDate(LocalDate.of(1913, 3, 1)).precision(DatePrecision.MONTH)
+                .documents(new HashSet<>(Set.of(shared)))
+                .build();
+        TimelineEvent later = TimelineEvent.builder()
+                .id(UUID.fromString("00000000-0000-0000-0000-000000000002"))
+                .title("Spätes Ereignis").type(EventType.PERSONAL)
+                .eventDate(LocalDate.of(1914, 3, 1)).precision(DatePrecision.MONTH)
+                .documents(new HashSet<>(Set.of(shared)))
+                .build();
+        when(timelineEventService.assembleDerivedEvents()).thenReturn(List.of());
+        when(documentService.getAllForTimeline()).thenReturn(List.of(shared));
+
+        // `later` first in iteration: a naive putIfAbsent would wrongly pick it.
+        when(eventRepository.findAll()).thenReturn(List.of(later, earlier));
+        assertThat(theLetter(timelineService.assemble(noFilters())).linkedEventId())
+                .isEqualTo(earlier.getId());
+
+        // Reversed order yields the same winner — the link is order-independent.
+        when(eventRepository.findAll()).thenReturn(List.of(earlier, later));
+        assertThat(theLetter(timelineService.assemble(noFilters())).linkedEventId())
+                .isEqualTo(earlier.getId());
+    }
+
+    @Test
+    void letter_linked_only_to_a_filtered_out_event_has_null_linkedEventId() {
+        // finding #10: the link pass runs over the events that survived the filter, not all of
+        // them. A letter whose only linking event is excluded by the active filter links to
+        // nothing (the frontend would drop it loose anyway) — and the lazy `documents` collection
+        // is never hydrated for events that are off-screen.
+        Document letterDoc = docWithDate(LocalDate.of(1916, 5, 1), DatePrecision.MONTH);
+        TimelineEvent worldEvent = TimelineEvent.builder().id(UUID.randomUUID())
+                .title("Somme").type(EventType.HISTORICAL)
+                .documents(new HashSet<>(Set.of(letterDoc)))
+                .build();
+        when(eventRepository.findAll()).thenReturn(List.of(worldEvent));
+        when(timelineEventService.assembleDerivedEvents()).thenReturn(List.of());
+        when(documentService.getAllForTimeline()).thenReturn(List.of(letterDoc));
+
+        // Filter to PERSONAL only → the HISTORICAL event is filtered out of the view.
+        TimelineEntryDTO entry = theLetter(timelineService.assemble(
+                new TimelineFilter(null, null, EventType.PERSONAL, null, null)));
+
+        assertThat(entry.linkedEventId()).isNull();
+    }
+
+    private static TimelineEntryDTO theLetter(TimelineDTO result) {
+        return java.util.stream.Stream.concat(
+                        result.years().stream().flatMap(y -> y.entries().stream()),
+                        result.undated().stream())
+                .filter(e -> e.kind() == Kind.LETTER)
+                .findFirst().orElseThrow();
+    }
+
     private static TimelineEntryDTO onlyLetterEntry(TimelineDTO result) {
         assertThat(result.years()).hasSize(1);
         return result.years().get(0).entries().get(0);
@@ -523,7 +623,7 @@ class TimelineServiceTest {
     private static TimelineEntryDTO letter(LocalDate date, DatePrecision precision, String title) {
         return new TimelineEntryDTO(Kind.LETTER, precision, false, "", "",
                 date, null, title, null, null, UUID.randomUUID(), List.of(), null,
-                null, null, null);
+                null, null, null, null);
     }
 
     private static Document docWithDate(LocalDate date, DatePrecision precision) {

frontend/messages/de.json

@@ -1049,10 +1049,12 @@
 	"timeline_derived_birth": "Geburt",
 	"timeline_derived_death": "Tod",
 	"timeline_derived_marriage": "Heirat",
-	"timeline_grouping_date": "Gruppierung: Datum",
 	"timeline_provenance_derived": "abgeleitet",
 	"timeline_provenance_curated": "kuratiert",
+	"timeline_bucket_show_more": "+ {count} weitere Briefe anzeigen",
+	"timeline_bucket_show_less": "Weniger anzeigen",
 	"timeline_letter_glyph_label": "Brief",
+	"timeline_cluster_letter_count": "{count} Briefe",
 	"timeline_tag_chip_label": "Thema",
 	"timeline_layer_historical_suffix": "historisch",
 	"timeline_strip_density_caption": "Monats-Dichte",

frontend/messages/en.json

@@ -1049,10 +1049,12 @@
 	"timeline_derived_birth": "Birth",
 	"timeline_derived_death": "Death",
 	"timeline_derived_marriage": "Marriage",
-	"timeline_grouping_date": "Grouping: Date",
 	"timeline_provenance_derived": "derived",
 	"timeline_provenance_curated": "curated",
+	"timeline_bucket_show_more": "+ {count} more letters",
+	"timeline_bucket_show_less": "Show fewer",
 	"timeline_letter_glyph_label": "Letter",
+	"timeline_cluster_letter_count": "{count} letters",
 	"timeline_tag_chip_label": "Topic",
 	"timeline_layer_historical_suffix": "historical",
 	"timeline_strip_density_caption": "Monthly density",

frontend/messages/es.json

@@ -1049,10 +1049,12 @@
 	"timeline_derived_birth": "Nacimiento",
 	"timeline_derived_death": "Fallecimiento",
 	"timeline_derived_marriage": "Matrimonio",
-	"timeline_grouping_date": "Agrupación: Fecha",
 	"timeline_provenance_derived": "derivado",
 	"timeline_provenance_curated": "curado",
+	"timeline_bucket_show_more": "+ {count} cartas más",
+	"timeline_bucket_show_less": "Mostrar menos",
 	"timeline_letter_glyph_label": "Carta",
+	"timeline_cluster_letter_count": "{count} cartas",
 	"timeline_tag_chip_label": "Tema",
 	"timeline_layer_historical_suffix": "histórico",
 	"timeline_strip_density_caption": "Densidad mensual",

frontend/src/lib/generated/api.ts

@@ -2467,6 +2467,8 @@ export interface components {
             rootTagId?: string;
             rootTagName?: string;
             rootTagColor?: string;
+            /** Format: uuid */
+            linkedEventId?: string;
         };
         TimelineYearDTO: {
             /** Format: int32 */

frontend/src/lib/messages.spec.ts

@@ -74,9 +74,10 @@ describe('message key parity', () => {
 	// every locale so no surface ever falls back to a missing translation.
 	it('zeitstrahl visual-fidelity keys are present in all locales (#833 REQ-015)', () => {
 		const requiredKeys = [
-			'timeline_grouping_date',
 			'timeline_provenance_derived',
 			'timeline_provenance_curated',
+			'timeline_bucket_show_more',
+			'timeline_bucket_show_less',
 			'timeline_letter_glyph_label',
 			'timeline_layer_historical_suffix',
 			'timeline_strip_density_caption',
@@ -99,6 +100,14 @@ describe('message key parity', () => {
 		expect(es).toMatchObject({ timeline_tag_chip_label: 'Tema' });
 	});
 
+	// #850 finding #6: the event-card letter count carries an sr-only "{count} Briefe" so the
+	// bare "· 2" never announces to a screen reader without context.
+	it('zeitstrahl cluster letter-count key is present in all locales (#850 finding #6)', () => {
+		expect(de).toHaveProperty('timeline_cluster_letter_count');
+		expect(en).toHaveProperty('timeline_cluster_letter_count');
+		expect(es).toHaveProperty('timeline_cluster_letter_count');
+	});
+
 	// #780 REQ-010: the layer-filter strings are Paraglide keys in every locale.
 	// timeline_filter_trigger (0 active) and timeline_filter_trigger_active ({count},
 	// ≥1 active) are distinct keys so the trigger never reads "Filter (0 aktiv)".

frontend/src/lib/timeline/EventCluster.svelte

@@ -0,0 +1,104 @@
+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import LetterCard from './LetterCard.svelte';
+import GlyphLabel from './GlyphLabel.svelte';
+import EventHeader from './EventHeader.svelte';
+import { entryKey } from './entryKey';
+import { CLUSTER_PREVIEW } from './eventClustering';
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+/**
+ * A curated event with linked letters, rendered as one contained card: the event IS the card's
+ * header (so its title reads once — never also as a floating pill, #850 REQ-002), and its letters
+ * sit inside as compact `.lcard.ev` cards.
+ *
+ * - Same-year event (`event` given): the shared EventHeader carries the accent glyph + sr-only
+ *   label, the title, a `{date} · {kuratiert|abgeleitet}` subtitle, the letter count, and — for a
+ *   curator on a curated event — an edit link to `/zeitstrahl/events/{eventId}/edit` (REQ-002).
+ * - Cross-year (`title` given, no `event`): a plain `✉ {title}` text header, no edit link, no pill
+ *   chrome — it holds that other year's linked letters (REQ-004).
+ *
+ * A card shows its first {@link CLUSTER_PREVIEW} letters, then a keyboard-operable show-more/less
+ * toggle reveals/collapses the rest instead of flooding the timeline (REQ-003).
+ */
+let {
+	letters,
+	event = undefined,
+	title = '',
+	canWrite = false
+}: {
+	letters: TimelineEntryDTO[];
+	/** The same-year curated event whose letters this card holds — renders as the header. */
+	event?: TimelineEntryDTO;
+	/** Header label for a cross-year card (no `event`). */
+	title?: string;
+	canWrite?: boolean;
+} = $props();
+
+const count = $derived(letters.length);
+
+// First-5 preview + show-more (REQ-003): a large cluster stays readable instead of dumping every
+// card into the timeline.
+let expanded = $state(false);
+const visible = $derived(expanded ? letters : letters.slice(0, CLUSTER_PREVIEW));
+const hiddenCount = $derived(letters.length - CLUSTER_PREVIEW);
+</script>
+
+<section
+	class="my-3 overflow-hidden rounded-md border border-l-2 border-line border-l-brand-mint bg-surface shadow-sm"
+	data-testid="event-card"
+>
+	{#if event}
+		<!-- A same-year curated event IS the card header (the shared EventHeader) — its title reads
+		     once here, never also as a floating pill (REQ-002); the edit pencil uses the single
+		     canEditEvent gate (REQ-010, #850 finding #5). -->
+		<header
+			data-testid="event-header"
+			class="flex items-center gap-2 border-b border-line bg-canvas px-3 py-2"
+		>
+			<EventHeader entry={event} canWrite={canWrite} count={count} />
+		</header>
+	{:else}
+		<!-- Cross-year card (REQ-004): the same event's letters in another year band get a plain
+		     ✉ text header — no pill chrome, no edit link. -->
+		<header
+			data-testid="event-header"
+			class="flex items-center gap-2 border-b border-line px-3 py-2"
+		>
+			<span class="font-serif text-sm font-bold whitespace-pre-line text-ink">
+				<GlyphLabel glyph="✉" label={m.timeline_letter_glyph_label()} />
+				{title}
+			</span>
+			<span data-testid="event-count" class="font-sans text-xs text-ink-3">
+				<span aria-hidden="true">· {count}</span>
+				<span class="sr-only">{m.timeline_cluster_letter_count({ count })}</span>
+			</span>
+		</header>
+	{/if}
+
+	<div class="px-3 py-2">
+		<ul class="space-y-1.5">
+			{#each visible as letter (entryKey(letter))}
+				<li>
+					<LetterCard entry={letter} variant="event" compact={true} />
+				</li>
+			{/each}
+		</ul>
+		{#if hiddenCount > 0}
+			<button
+				type="button"
+				data-testid="bucket-show-more"
+				aria-expanded={expanded}
+				onclick={() => (expanded = !expanded)}
+				style="display: inline-flex; align-items: center; min-height: 44px"
+				class="mt-1 px-1 font-sans text-xs font-semibold text-brand-navy hover:underline focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
+			>
+				{expanded
+					? m.timeline_bucket_show_less()
+					: m.timeline_bucket_show_more({ count: hiddenCount })}
+			</button>
+		{/if}
+	</div>
+</section>

frontend/src/lib/timeline/EventCluster.svelte.spec.ts

@@ -0,0 +1,129 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
+import * as m from '$lib/paraglide/messages.js';
+import EventCluster from './EventCluster.svelte';
+import { makeEntry } from './test-factories';
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+afterEach(() => cleanup());
+
+const EV_ID = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa';
+
+const makeEvent = (overrides: Partial<TimelineEntryDTO> = {}): TimelineEntryDTO =>
+	makeEntry({
+		kind: 'EVENT',
+		type: 'PERSONAL',
+		documentId: undefined,
+		eventId: EV_ID,
+		eventDate: '1916-07-06',
+		precision: 'DAY',
+		title: 'Ein gewaltiger Stadtbrand',
+		...overrides
+	});
+
+const letters = (n: number): TimelineEntryDTO[] =>
+	Array.from({ length: n }, (_, i) =>
+		makeEntry({ kind: 'LETTER', documentId: `doc-${i}`, title: `Brief ${i}`, linkedEventId: EV_ID })
+	);
+
+describe('EventCluster — contained event card (#850)', () => {
+	it('renders a data-testid event-card with the event title once (REQ-002)', () => {
+		render(EventCluster, { letters: letters(2), event: makeEvent() });
+		expect(document.querySelector('[data-testid="event-card"]')).not.toBeNull();
+		const occurrences = (document.body.textContent?.match(/Ein gewaltiger Stadtbrand/g) ?? [])
+			.length;
+		expect(occurrences).toBe(1);
+	});
+
+	it('shows the event-edit link for a curator on a curated event (REQ-002)', () => {
+		render(EventCluster, { letters: letters(2), event: makeEvent(), canWrite: true });
+		const edit = document.querySelector('[data-testid="event-edit"]') as HTMLAnchorElement;
+		expect(edit).not.toBeNull();
+		expect(edit.getAttribute('href')).toBe(`/zeitstrahl/events/${EV_ID}/edit`);
+	});
+
+	it('hides the event-edit link when canWrite is false', () => {
+		render(EventCluster, { letters: letters(2), event: makeEvent(), canWrite: false });
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+	});
+
+	it('hides the event-edit link for a derived event even with canWrite', () => {
+		render(EventCluster, {
+			letters: letters(2),
+			event: makeEvent({ derived: true, eventId: undefined, derivedType: 'BIRTH' }),
+			canWrite: true
+		});
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+	});
+
+	it('renders its letters as compact a.lcard.ev cards (REQ-002)', () => {
+		render(EventCluster, { letters: letters(2), event: makeEvent() });
+		expect(document.querySelectorAll('a.lcard.ev').length).toBeGreaterThan(0);
+	});
+
+	it('shows the first 5 of 8 letters + a show-more toggle that expands to 8 then back to 5 (REQ-003)', async () => {
+		render(EventCluster, { letters: letters(8), event: makeEvent() });
+		expect(document.querySelectorAll('a.lcard').length).toBe(5);
+		const toggle = document.querySelector('[data-testid="bucket-show-more"]') as HTMLButtonElement;
+		expect(toggle).not.toBeNull();
+		expect(toggle.getAttribute('aria-expanded')).toBe('false');
+		expect(toggle.getBoundingClientRect().height).toBeGreaterThanOrEqual(44);
+
+		toggle.click();
+		await tick();
+		expect(document.querySelectorAll('a.lcard').length).toBe(8);
+		expect(toggle.getAttribute('aria-expanded')).toBe('true');
+
+		toggle.click();
+		await tick();
+		expect(document.querySelectorAll('a.lcard').length).toBe(5);
+	});
+
+	it('renders no show-more toggle when the cluster holds 5 or fewer letters', () => {
+		render(EventCluster, { letters: letters(5), event: makeEvent() });
+		expect(document.querySelector('[data-testid="bucket-show-more"]')).toBeNull();
+	});
+
+	it('renders a cross-year text header (✉ title, no event-edit, no pill) when no event is given (REQ-004)', () => {
+		render(EventCluster, {
+			letters: letters(2),
+			title: 'Briefe von der Front',
+			canWrite: true
+		});
+		expect(document.querySelector('[data-testid="event-card"]')).not.toBeNull();
+		expect(document.body.textContent).toContain('✉');
+		expect(document.body.textContent).toContain('Briefe von der Front');
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+	});
+
+	it('pairs the cross-year ✉ glyph with an sr-only label so it is not a silent glyph (finding #6)', () => {
+		render(EventCluster, { letters: letters(2), title: 'Briefe von der Front' });
+		const header = document.querySelector('[data-testid="event-header"]') as HTMLElement;
+		const hidden = header.querySelector('[aria-hidden="true"]');
+		expect(hidden?.textContent).toContain('✉');
+		const srOnly = header.querySelector('.sr-only');
+		expect(srOnly?.textContent).toBe(m.timeline_letter_glyph_label());
+	});
+
+	it('gives the letter count an sr-only "{count} Briefe" label so "· 2" is not announced bare (finding #6)', () => {
+		render(EventCluster, { letters: letters(2), title: 'Briefe von der Front' });
+		const count = document.querySelector('[data-testid="event-count"]') as HTMLElement;
+		// the visible "· 2" stays aria-hidden; the sr-only sibling carries the meaning
+		expect(count.querySelector('[aria-hidden="true"]')?.textContent).toContain('· 2');
+		expect(count.querySelector('.sr-only')?.textContent).toBe(
+			m.timeline_cluster_letter_count({ count: 2 })
+		);
+	});
+
+	it('renders an HTML-bearing event title verbatim as text, never as markup (REQ-010)', () => {
+		render(EventCluster, {
+			letters: letters(1),
+			event: makeEvent({ title: '<img src=x onerror=alert(1)>' })
+		});
+		expect(document.querySelector('[data-testid="event-card"] img')).toBeNull();
+		expect(document.body.textContent).toContain('<img src=x onerror=alert(1)>');
+	});
+});

frontend/src/lib/timeline/EventHeader.svelte

@@ -0,0 +1,69 @@
+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import GlyphLabel from './GlyphLabel.svelte';
+import { getAccentConfig, canEditEvent } from './eventCardConfig';
+import { timelineDateLabel } from './dateLabel';
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+/**
+ * The shared header for a curated or derived timeline event — the accent glyph circle, the title,
+ * and the `{date} · {kuratiert|abgeleitet}` subtitle, plus a curator edit pencil gated by the
+ * single canEditEvent() contract. Rendered by EventPill (inside the floating axis pill) and by
+ * EventCluster (as a same-year event-card header), so the glyph/title/subtitle markup and the
+ * security-relevant edit gate live in one place (#850 finding #5). It renders three sibling nodes
+ * (glyph circle, text block, optional edit pencil) into the parent's flex row — the parent owns
+ * the wrapper (pill vs card header). An optional letter `count` appends a screen-reader-labeled
+ * "· {count}" for the event-card case.
+ */
+let {
+	entry,
+	canWrite = false,
+	count = undefined
+}: { entry: TimelineEntryDTO; canWrite?: boolean; count?: number } = $props();
+
+const config = $derived(getAccentConfig(entry));
+const dateLabel = $derived(timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd));
+// Provenance reads off entry.derived: a derived life-event is "abgeleitet", a curated event
+// "kuratiert"; the date is an optional prefix so an undated event still reads the provenance.
+const provenance = $derived(
+	entry.derived ? m.timeline_provenance_derived() : m.timeline_provenance_curated()
+);
+const subtitle = $derived(dateLabel ? `${dateLabel} · ${provenance}` : provenance);
+const canEdit = $derived(canEditEvent(entry, canWrite));
+</script>
+
+<span
+	class="flex h-7 w-7 shrink-0 items-center justify-center rounded-full {config.accent === 'curated'
+		? 'bg-brand-mint text-brand-navy'
+		: 'bg-brand-navy text-brand-mint'}"
+>
+	<GlyphLabel glyph={config.glyph} label={config.label} />
+</span>
+<span class="min-w-0 text-left">
+	{#if entry.title}
+		<span class="block font-serif text-sm font-bold whitespace-pre-line text-brand-navy"
+			>{entry.title}</span
+		>
+	{/if}
+	<span class="block font-sans text-xs text-ink-3">
+		{subtitle}
+		{#if count !== undefined}
+			<span data-testid="event-count">
+				<span aria-hidden="true">· {count}</span>
+				<span class="sr-only">{m.timeline_cluster_letter_count({ count })}</span>
+			</span>
+		{/if}
+	</span>
+</span>
+{#if canEdit}
+	<a
+		data-testid="event-edit"
+		href="/zeitstrahl/events/{entry.eventId}/edit"
+		class="ml-auto rounded-sm px-1 font-sans text-xs text-ink-3 hover:text-brand-navy focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
+	>
+		<span aria-hidden="true">✎</span>
+		<span class="sr-only">{m.btn_edit()}</span>
+	</a>
+{/if}

frontend/src/lib/timeline/EventHeader.svelte.spec.ts

@@ -0,0 +1,66 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import * as m from '$lib/paraglide/messages.js';
+import EventHeader from './EventHeader.svelte';
+import { makeEntry } from './test-factories';
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+afterEach(() => cleanup());
+
+const EV_ID = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa';
+
+const curated = (overrides: Partial<TimelineEntryDTO> = {}): TimelineEntryDTO =>
+	makeEntry({
+		kind: 'EVENT',
+		type: 'PERSONAL',
+		derived: false,
+		eventId: EV_ID,
+		eventDate: '1916-07-06',
+		precision: 'DAY',
+		title: 'Ein gewaltiger Stadtbrand',
+		documentId: undefined,
+		...overrides
+	});
+
+describe('EventHeader', () => {
+	it('renders the glyph with an sr-only label, the title, and the provenance subtitle', () => {
+		render(EventHeader, { entry: curated() });
+		expect(document.querySelector('.sr-only')?.textContent).toBe(m.timeline_layer_family());
+		expect(document.body.textContent).toContain('Ein gewaltiger Stadtbrand');
+		expect(document.body.textContent).toContain(m.timeline_provenance_curated());
+	});
+
+	it('shows the edit pencil for a writer on a curated event (canEditEvent gate)', () => {
+		render(EventHeader, { entry: curated(), canWrite: true });
+		const edit = document.querySelector('[data-testid="event-edit"]') as HTMLAnchorElement;
+		expect(edit).not.toBeNull();
+		expect(edit.getAttribute('href')).toBe(`/zeitstrahl/events/${EV_ID}/edit`);
+	});
+
+	it('hides the edit pencil without write, for a derived event, and for a null eventId', () => {
+		render(EventHeader, { entry: curated(), canWrite: false });
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+		cleanup();
+		render(EventHeader, { entry: curated({ derived: true }), canWrite: true });
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+		cleanup();
+		render(EventHeader, { entry: curated({ eventId: undefined }), canWrite: true });
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+	});
+
+	it('renders a screen-reader-labeled letter count when a count is given', () => {
+		render(EventHeader, { entry: curated(), count: 3 });
+		const count = document.querySelector('[data-testid="event-count"]') as HTMLElement;
+		expect(count.querySelector('[aria-hidden="true"]')?.textContent).toContain('· 3');
+		expect(count.querySelector('.sr-only')?.textContent).toBe(
+			m.timeline_cluster_letter_count({ count: 3 })
+		);
+	});
+
+	it('omits the letter count when no count is given (the pill case)', () => {
+		render(EventHeader, { entry: curated() });
+		expect(document.querySelector('[data-testid="event-count"]')).toBeNull();
+	});
+});

frontend/src/lib/timeline/EventPill.svelte

@@ -1,32 +1,21 @@
 <script lang="ts">
-import * as m from '$lib/paraglide/messages.js';
+import EventHeader from './EventHeader.svelte';
 import { getAccentConfig } from './eventCardConfig';
-import { timelineDateLabel } from './dateLabel';
 import type { components } from '$lib/generated/api';
 
 type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
 
 /**
  * Centered axis pill for a derived life-event or a curated PERSONAL event
- * (REQ-007/008). The glyph is wrapped aria-hidden with an sr-only label sibling
- * (REQ-018). An edit affordance shows only for a curated event with an eventId
- * (never derived, never null — REQ-008) and only for a curator who holds
- * WRITE_ALL (`canWrite`, gate-closed by default — #842 REQ-005/007/008). The
- * gate is UX only; the real boundary is the #781 route guard + backend permission.
+ * (REQ-007/008). The pill border keys off the accent (curated = mint, derived =
+ * navy); its glyph, title, subtitle, and curator edit pencil are the shared
+ * EventHeader, so the edit gate (canEditEvent) lives in one place — #842
+ * REQ-005/007/008, #850 finding #5. The gate is UX only; the real boundary is the
+ * #781 route guard + backend permission.
  */
 let { entry, canWrite = false }: { entry: TimelineEntryDTO; canWrite?: boolean } = $props();
 
 const config = $derived(getAccentConfig(entry));
-const dateLabel = $derived(timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd));
-// Provenance reads off entry.derived (not the accent): a derived life-event is
-// "abgeleitet", a curated PERSONAL event is "kuratiert" (REQ-007).
-const provenance = $derived(
-	entry.derived ? m.timeline_provenance_derived() : m.timeline_provenance_curated()
-);
-// Provenance always shows; the date is an optional prefix so an undated event
-// still reads "abgeleitet"/"kuratiert" (REQ-007).
-const subtitle = $derived(dateLabel ? `${dateLabel} · ${provenance}` : provenance);
-const canEdit = $derived(canWrite && !entry.derived && entry.eventId != null);
 </script>
 
 <div class="flex justify-center">
@@ -36,32 +25,6 @@ const canEdit = $derived(canWrite && !entry.derived && entry.eventId != null);
 			? 'border-2 border-brand-mint'
 			: 'border border-brand-navy'}"
 	>
-		<span
-			class="flex h-7 w-7 shrink-0 items-center justify-center rounded-full {config.accent ===
-			'curated'
-				? 'bg-brand-mint text-brand-navy'
-				: 'bg-brand-navy text-brand-mint'}"
-		>
-			<span aria-hidden="true">{config.glyph}</span>
-			<span class="sr-only">{config.label}</span>
-		</span>
-		<span class="text-left">
-			{#if entry.title}
-				<span class="block font-serif text-sm font-bold whitespace-pre-line text-brand-navy"
-					>{entry.title}</span
-				>
-			{/if}
-			<span class="block font-sans text-xs text-ink-3">{subtitle}</span>
-		</span>
-		{#if canEdit}
-			<a
-				data-testid="event-edit"
-				href="/zeitstrahl/events/{entry.eventId}/edit"
-				class="ml-1 rounded-sm px-1 font-sans text-xs text-ink-3 hover:text-brand-navy focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
-			>
-				<span aria-hidden="true">✎</span>
-				<span class="sr-only">{m.btn_edit()}</span>
-			</a>
-		{/if}
+		<EventHeader entry={entry} canWrite={canWrite} />
 	</div>
 </div>

frontend/src/lib/timeline/LetterCard.svelte

@@ -11,11 +11,33 @@ type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
  * A single archive letter on the timeline: sender → receiver, title, and a
  * precision-aware date chip, linking to the document. Names/titles are
  * OCR/import-derived — rendered via default `{...}` escaping with
- * `whitespace-pre-line` for line breaks (REQ-021); never the raw-HTML directive.
+ * `whitespace-pre-line` for line breaks (REQ-010); never the raw-HTML directive.
+ *
+ * Inside an event cluster the card sits in the contained event card and renders as
+ * the `.lcard.ev` `compact` variant (#850, REQ-002): tighter row, and the redundant
+ * date chip is dropped when the title already embeds the date. The per-letter tag
+ * chip can be suppressed via `suppressTagChip` for callers that already convey it.
  */
-let { entry }: { entry: TimelineEntryDTO } = $props();
+let {
+	entry,
+	variant = 'plain',
+	suppressTagChip = false,
+	compact = false
+}: {
+	entry: TimelineEntryDTO;
+	variant?: 'plain' | 'event';
+	suppressTagChip?: boolean;
+	compact?: boolean;
+} = $props();
 
+const isEventVariant = $derived(variant === 'event');
 const dateLabel = $derived(timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd));
+// Inside an event card the band frames the time, so a compact in-card letter drops the
+// redundant date chip — but ONLY when the (free-form OCR) title actually embeds the formatted
+// date, e.g. "H-0023 – 6. Juli 1916". A title without the date keeps its chip, so a letter like
+// "Brief an Mutter" never loses its month/day (the band frames only the year) — #850, finding #4.
+const titleEmbedsDate = $derived(!!dateLabel && !!entry.title && entry.title.includes(dateLabel));
+const showDate = $derived(!compact || !titleEmbedsDate);
 const sender = $derived(entry.senderName === '' ? m.timeline_unknown_person() : entry.senderName);
 const receiver = $derived(
 	entry.receiverName === '' ? m.timeline_unknown_person() : entry.receiverName
@@ -28,28 +50,37 @@ const receiver = $derived(
 <a
 	href="/documents/{entry.documentId}"
 	style="display: flex; flex-direction: column; justify-content: center; min-height: 44px"
-	class="rounded-sm border border-l-[3px] border-line border-l-brand-mint bg-surface px-3 py-2 shadow-sm transition-colors hover:border-brand-mint focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
+	class="lcard rounded-sm border border-l-[3px] border-line border-l-brand-mint bg-surface px-3 shadow-sm transition-colors hover:border-brand-mint focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
+	class:py-2={!compact}
+	class:py-1={compact}
+	class:ev={isEventVariant}
+	class:compact={compact}
 >
 	{#if entry.title}
 		<!-- ✉ + sr-only label are static chrome rendered as sibling nodes, NEVER
 		     interpolated into the escaped user title; the title keeps its own
 		     pre-line span for multi-line OCR text (REQ-008/016/021). -->
-		<span class="font-serif text-sm font-bold break-words text-ink">
+		<span
+			class="font-serif font-bold break-words text-ink"
+			class:text-sm={!compact}
+			class:text-xs={compact}
+		>
 			<GlyphLabel glyph="✉" label={m.timeline_letter_glyph_label()} />
 			<span class="whitespace-pre-line">{entry.title}</span>
 		</span>
 	{/if}
-	<span class="mt-0.5 font-sans text-xs break-words text-ink-3">
+	<span class="font-sans text-xs break-words text-ink-3" class:mt-0.5={!compact}>
 		<span class="font-serif whitespace-pre-line">{sender}</span>
 		<span aria-hidden="true">→</span>
 		<span class="font-serif whitespace-pre-line">{receiver}</span>
-		{#if dateLabel}
+		{#if dateLabel && showDate}
 			<span data-testid="letter-date"> · {dateLabel}</span>
 		{/if}
 	</span>
-	{#if entry.rootTagName}
+	{#if entry.rootTagName && !suppressTagChip}
 		<!-- The primary root-tag chip sits on its own line beneath the meta line
-		     (#835 §3); absent when the letter has no tag (REQ-005). -->
+		     (#835 §3); absent when the letter has no tag (REQ-006), and suppressed when
+		     the caller already conveys the topic (suppressTagChip). -->
 		<TagChip name={entry.rootTagName} color={entry.rootTagColor ?? null} />
 	{/if}
 </a>

frontend/src/lib/timeline/LetterCard.svelte.spec.ts

@@ -127,3 +127,58 @@ describe('LetterCard', () => {
 		expect(chip?.textContent).toContain('Familie');
 	});
 });
+
+describe('LetterCard — event-cluster variants (#850, REQ-002)', () => {
+	it('carries the .lcard.ev class in the event variant (REQ-002)', () => {
+		render(LetterCard, { entry: makeEntry(), variant: 'event' });
+		expect(document.querySelector('a.lcard.ev')).not.toBeNull();
+	});
+
+	it('is a plain card with no .ev marker by default (REQ-006)', () => {
+		render(LetterCard, { entry: makeEntry() });
+		expect(document.querySelector('a.ev')).toBeNull();
+	});
+
+	it('suppresses the per-letter tag chip when asked, even with a root tag', () => {
+		render(LetterCard, {
+			entry: makeEntry({ rootTagName: 'Krieg', rootTagColor: 'sienna' }),
+			suppressTagChip: true
+		});
+		expect(document.querySelector('[data-testid="tag-chip"]')).toBeNull();
+	});
+
+	it('still shows the per-letter tag chip when not suppressed', () => {
+		render(LetterCard, { entry: makeEntry({ rootTagName: 'Krieg', rootTagColor: 'sienna' }) });
+		expect(document.querySelector('[data-testid="tag-chip"]')).not.toBeNull();
+	});
+
+	it('drops the compact date chip only when the title actually embeds the formatted date (#850)', () => {
+		// An archive title like "H-0023 – 6. Juli 1916" already carries the date, so inside an
+		// event card (where the band frames the time) the redundant chip is dropped.
+		const entry = makeEntry({ eventDate: '1916-07-06', precision: 'DAY' });
+		const dateLabel = timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd);
+		render(LetterCard, { entry: { ...entry, title: `H-0023 – ${dateLabel}` }, compact: true });
+		expect(document.querySelector('[data-testid="letter-date"]')).toBeNull();
+		expect(document.body.textContent).toContain('Karl Raddatz'); // sender still shown
+	});
+
+	it('keeps the compact date chip when the title does NOT embed the date (#850, finding #4)', () => {
+		// Titles are free-form OCR text — a titled letter whose title carries no date must keep
+		// its month/day, since inside an event card the band frames only the year.
+		render(LetterCard, {
+			entry: makeEntry({ eventDate: '1916-07-06', precision: 'DAY', title: 'Brief an Mutter' }),
+			compact: true
+		});
+		expect(document.querySelector('[data-testid="letter-date"]')).not.toBeNull();
+	});
+
+	it('keeps the date in the compact variant when the letter has no title (#850)', () => {
+		render(LetterCard, { entry: makeEntry({ title: undefined }), compact: true });
+		expect(document.querySelector('[data-testid="letter-date"]')).not.toBeNull();
+	});
+
+	it('renders the compact variant on a single tighter row (#850)', () => {
+		render(LetterCard, { entry: makeEntry(), compact: true });
+		expect(document.querySelector('a.lcard.compact')).not.toBeNull();
+	});
+});

frontend/src/lib/timeline/TimelineView.svelte

@@ -6,6 +6,7 @@ import LetterCard from './LetterCard.svelte';
 import EventPill from './EventPill.svelte';
 import WorldBand from './WorldBand.svelte';
 import { entryKey } from './entryKey';
+import { buildEventLookup } from './eventClustering';
 import type { components } from '$lib/generated/api';
 
 type TimelineDTO = components['schemas']['TimelineDTO'];
@@ -18,6 +19,11 @@ type TimelineYearDTO = components['schemas']['TimelineYearDTO'];
  * empty timeline shows a calm message (REQ-017). `personId` is a declared seam
  * for the per-person rail (issue #10) and is undefined here; it is not passed to
  * leaf cards (REQ-025). Owns no <main> — the layout does.
+ *
+ * The event lookup is built once over the whole (already layer-filtered) timeline
+ * and threaded to every band so a curated event's letters cluster under it inline
+ * (#850, REQ-002). The undated bucket stays plain (events as pills, letters as
+ * cards) — out of clustering scope.
  */
 let {
 	timeline,
@@ -25,6 +31,8 @@ let {
 	canWrite = false
 }: { timeline: TimelineDTO; personId?: string; canWrite?: boolean } = $props();
 
+const eventLookup = $derived(buildEventLookup(timeline));
+
 type Row = { t: 'band'; year: TimelineYearDTO } | { t: 'gap'; from: number; to: number };
 
 const rows = $derived.by<Row[]>(() => {
@@ -54,7 +62,7 @@ const isEmpty = $derived(timeline.years.length === 0 && timeline.undated.length
 		{#each rows as row (row.t === 'band' ? `band-${row.year.year}` : `gap-${row.from}`)}
 			<li>
 				{#if row.t === 'band'}
-					<YearBand year={row.year} canWrite={canWrite} />
+					<YearBand year={row.year} canWrite={canWrite} eventLookup={eventLookup} />
 				{:else}
 					<GapSpan from={row.from} to={row.to} />
 				{/if}

frontend/src/lib/timeline/TimelineView.svelte.spec.ts

@@ -341,4 +341,67 @@ describe('TimelineView', () => {
 		expect(hrefs).toContain('/zeitstrahl/events/wb/edit');
 		expect(hrefs).toContain('/zeitstrahl/events/wu/edit');
 	});
+
+	it('builds the event lookup and clusters a curated event + same-year linked letter into an event-card (#850)', () => {
+		const evId = 'eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee';
+		const event = makeEntry({
+			kind: 'EVENT',
+			type: 'PERSONAL',
+			derived: false,
+			eventId: evId,
+			eventDate: '1916-07-06',
+			precision: 'DAY',
+			title: 'Ein gewaltiger Stadtbrand',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined
+		});
+		const letter = makeEntry({
+			eventDate: '1916-05-10',
+			documentId: 'doc-linked',
+			title: 'Brief',
+			linkedEventId: evId
+		});
+		render(TimelineView, {
+			timeline: makeTimelineDTO({ years: [makeYear(1916, [event, letter])] })
+		});
+		expect(document.querySelector('[data-testid="event-card"]')).not.toBeNull();
+		expect(document.querySelector('a.lcard.ev')).not.toBeNull();
+		// the title reads once — the event is the card header, not also a loose pill
+		const titles = (document.body.textContent?.match(/Ein gewaltiger Stadtbrand/g) ?? []).length;
+		expect(titles).toBe(1);
+	});
+
+	it('keeps a HISTORICAL event a WorldBand with a same-year linked letter — never clusters (REQ-014)', () => {
+		const evId = 'dddddddd-dddd-dddd-dddd-dddddddddddd';
+		const world = makeEntry({
+			kind: 'EVENT',
+			type: 'HISTORICAL',
+			derived: false,
+			eventId: evId,
+			eventDate: '1916-07-01',
+			precision: 'DAY',
+			title: 'Schlacht an der Somme',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined
+		});
+		const letter = makeEntry({
+			eventDate: '1916-05-10',
+			documentId: 'doc-world-linked',
+			title: 'Brief von der Front',
+			linkedEventId: evId
+		});
+		render(TimelineView, {
+			timeline: makeTimelineDTO({ years: [makeYear(1916, [world, letter])] })
+		});
+		// the world event stays a full-width band — no contained event card
+		expect(document.querySelector('[data-testid="event-card"]')).toBeNull();
+		expect(document.querySelector('a.lcard.ev')).toBeNull();
+		// the linked letter renders loose on the spine, not inside a card
+		expect(document.querySelector('.letter-row')).not.toBeNull();
+		// and the band keeps its WorldBand "· historisch" register
+		expect(document.body.textContent).toContain(m.timeline_layer_historical_suffix());
+		expect(document.body.textContent).toContain('Schlacht an der Somme');
+	});
 });

frontend/src/lib/timeline/WorldBand.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import * as m from '$lib/paraglide/messages.js';
-import { getAccentConfig } from './eventCardConfig';
+import { getAccentConfig, canEditEvent } from './eventCardConfig';
 import { timelineDateLabel } from './dateLabel';
 import type { components } from '$lib/generated/api';
 
@@ -26,9 +26,9 @@ const dateText = $derived(showSpan ? null : entry.precision === 'RANGE' ? fromYe
 // Every WorldBand is a HISTORICAL band, so the visible "historisch" register
 // always trails the subtitle as plain text — never a second pill (REQ-009).
 const historical = $derived(m.timeline_layer_historical_suffix());
-// A HISTORICAL event is never derived, so the edit gate is just the curator
-// flag plus a real eventId (#842 REQ-006/008).
-const canEdit = $derived(canWrite && entry.eventId != null);
+// A HISTORICAL event is never derived, so canEditEvent's derived check is a
+// no-op here — the gate is the curator flag plus a real eventId (#842 REQ-006/008).
+const canEdit = $derived(canEditEvent(entry, canWrite));
 </script>
 
 <div class="my-3 border-y border-line bg-canvas px-4 py-2 text-center">

frontend/src/lib/timeline/YearBand.svelte

@@ -3,8 +3,10 @@ import EventPill from './EventPill.svelte';
 import WorldBand from './WorldBand.svelte';
 import LetterCard from './LetterCard.svelte';
 import YearLetterStrip from './YearLetterStrip.svelte';
+import EventCluster from './EventCluster.svelte';
 import { isDense } from './timelineDensity';
 import { entryKey } from './entryKey';
+import { splitYearLetters, type EventCluster as EventClusterModel } from './eventClustering';
 import type { components } from '$lib/generated/api';
 
 type TimelineYearDTO = components['schemas']['TimelineYearDTO'];
@@ -12,37 +14,113 @@ type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
 
 /**
  * One year of the timeline: a <section> with a sticky <h2> (REQ-006). Events
- * render in DTO order as pills/bands; letters render as individual cards while
- * the band holds ≤ 12 (REQ-011) or collapse to a single density strip above that
- * (REQ-012). Entries are never re-sorted — DTO order is preserved (REQ-003).
+ * render in DTO order as pills/world-bands; entries are never re-sorted (REQ-003).
+ *
+ * A curated event with letters linked to it (#850) becomes a contained event card:
+ * the event IS the card header and its linked letters sit inside (no separate pill —
+ * REQ-002). A curated event with letters in another year band renders here as a
+ * cross-year text-header card (REQ-004). An event with no linked letters stays a
+ * plain pill/world-band (REQ-005).
+ *
+ * Every other letter (no linkedEventId, or linking to an event the #780 layer filter
+ * removed) stays loose: alternating left/right while the band holds ≤ 12 such loose
+ * letters (REQ-006), folding into a single month-density strip above that (REQ-007).
+ * The loose-letter layout and the strip count ONLY these loose letters — clustered
+ * letters never re-appear loose (REQ-007).
  */
-let { year, canWrite = false }: { year: TimelineYearDTO; canWrite?: boolean } = $props();
+let {
+	year,
+	canWrite = false,
+	eventLookup
+}: {
+	year: TimelineYearDTO;
+	canWrite?: boolean;
+	eventLookup?: Map<string, string>;
+} = $props();
 
 type Row =
 	| { t: 'event'; entry: TimelineEntryDTO }
+	| { t: 'eventcard'; event?: TimelineEntryDTO; cluster: EventClusterModel }
 	| { t: 'letter'; entry: TimelineEntryDTO; side: 'left' | 'right' }
 	| { t: 'strip' };
 
-const letters = $derived(year.entries.filter((e) => e.kind === 'LETTER'));
-const dense = $derived(isDense(letters.length));
+// Split this band's letters into event clusters and the loose remainder once; the loose
+// list alone drives the alternating layout and the density strip (REQ-007).
+const split = $derived(
+	splitYearLetters(
+		year.entries.filter((e) => e.kind === 'LETTER'),
+		eventLookup
+	)
+);
+const loose = $derived(split.loose);
+const dense = $derived(isDense(loose.length));
+// Clusters keyed by eventId (built once in splitYearLetters): row assembly looks a letter's
+// disposition up in O(1) — `byEvent.has(linkedEventId)` — instead of scanning the loose array
+// per letter (was O(L²) on a dense band), and resolves an event's card with `byEvent.get`.
+const byEvent = $derived(split.byEvent);
+
+// Event ids that have a same-year EVENT entry in THIS band: those clusters render as that
+// event's header (at the EVENT position); every other cluster is cross-year (REQ-004/015).
+const sameYearEventIds = $derived.by<Record<string, true>>(() => {
+	const ids: Record<string, true> = {};
+	for (const entry of year.entries) {
+		if (entry.kind === 'EVENT' && entry.eventId) ids[entry.eventId] = true;
+	}
+	return ids;
+});
 
 const rows = $derived.by<Row[]>(() => {
 	const out: Row[] = [];
+	const emitted: Record<string, true> = {};
 	let stripInserted = false;
 	let letterIndex = 0;
+
 	for (const entry of year.entries) {
 		if (entry.kind === 'EVENT') {
-			out.push({ t: 'event', entry });
-		} else if (!dense) {
-			out.push({ t: 'letter', entry, side: letterIndex % 2 === 0 ? 'left' : 'right' });
-			letterIndex += 1;
-		} else if (!stripInserted) {
-			out.push({ t: 'strip' });
-			stripInserted = true;
+			// A curated event whose letters live in THIS band becomes the contained card's
+			// header — its title reads once, no separate pill (REQ-002). Otherwise it stays a
+			// plain pill/world-band (REQ-005).
+			const cluster = entry.eventId ? byEvent.get(entry.eventId) : undefined;
+			if (cluster) {
+				out.push({ t: 'eventcard', event: entry, cluster });
+				emitted[cluster.eventId] = true;
+			} else {
+				out.push({ t: 'event', entry });
+			}
+			continue;
+		}
+
+		const cluster = entry.linkedEventId ? byEvent.get(entry.linkedEventId) : undefined;
+		if (!cluster) {
+			// A loose letter (not clustered): alternate while sparse, or fold the whole loose set
+			// into one density strip (inserted once, at the first loose letter) when dense.
+			if (!dense) {
+				out.push({ t: 'letter', entry, side: letterIndex % 2 === 0 ? 'left' : 'right' });
+				letterIndex += 1;
+			} else if (!stripInserted) {
+				out.push({ t: 'strip' });
+				stripInserted = true;
+			}
+			continue;
+		}
+
+		// A clustered letter. A same-year cluster is emitted at its EVENT entry, so skip it here.
+		// A cross-year cluster has no EVENT anchor in this band — emit its ✉ card HERE, at the
+		// position of its earliest linked letter, so the band stays in strict time order (REQ-015).
+		if (!sameYearEventIds[cluster.eventId] && !emitted[cluster.eventId]) {
+			out.push({ t: 'eventcard', cluster });
+			emitted[cluster.eventId] = true;
 		}
 	}
+
 	return out;
 });
+
+function rowKey(row: Row): string {
+	if (row.t === 'strip') return `strip-${year.year}`;
+	if (row.t === 'eventcard') return `evcard:${row.cluster.eventId}`;
+	return entryKey(row.entry);
+}
 </script>
 
 <section class="py-2">
@@ -56,20 +134,27 @@ const rows = $derived.by<Row[]>(() => {
 	</h2>
 
 	<div class="mt-3 space-y-3">
-		{#each rows as row (row.t === 'strip' ? `strip-${year.year}` : entryKey(row.entry))}
+		{#each rows as row (rowKey(row))}
 			{#if row.t === 'event'}
 				{#if row.entry.type === 'HISTORICAL'}
 					<WorldBand entry={row.entry} canWrite={canWrite} />
 				{:else}
 					<EventPill entry={row.entry} canWrite={canWrite} />
 				{/if}
+			{:else if row.t === 'eventcard'}
+				<EventCluster
+					letters={row.cluster.letters}
+					event={row.event}
+					title={row.cluster.title}
+					canWrite={canWrite}
+				/>
 			{:else if row.t === 'letter'}
 				<div class="letter-row" data-side={row.side}>
 					<span data-testid="letter-dot" class="letter-dot bg-surface" aria-hidden="true"></span>
 					<LetterCard entry={row.entry} />
 				</div>
 			{:else}
-				<YearLetterStrip letters={letters} year={year.year} />
+				<YearLetterStrip letters={loose} year={year.year} />
 			{/if}
 		{/each}
 	</div>

frontend/src/lib/timeline/YearBand.svelte.spec.ts

@@ -165,3 +165,126 @@ describe('YearBand', () => {
 		}
 	});
 });
+
+describe('YearBand — inline event clustering (#850)', () => {
+	const EV_ID = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa';
+
+	function curatedEvent(overrides = {}) {
+		return makeEntry({
+			kind: 'EVENT',
+			derived: false,
+			type: 'PERSONAL',
+			eventId: EV_ID,
+			eventDate: '1916-07-06',
+			precision: 'DAY',
+			title: 'Ein gewaltiger Stadtbrand',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined,
+			...overrides
+		});
+	}
+
+	function linkedLetters(year: number, count: number, eventId = EV_ID) {
+		return Array.from({ length: count }, (_, i) =>
+			makeEntry({
+				eventDate: `${year}-05-10`,
+				documentId: `linked-${i}`,
+				title: `Brief ${i}`,
+				linkedEventId: eventId
+			})
+		);
+	}
+
+	const lookup = new Map([[EV_ID, 'Ein gewaltiger Stadtbrand']]);
+
+	it('renders a curated event with a same-year linked letter as one event-card, title once, no separate pill (REQ-002)', () => {
+		render(YearBand, {
+			year: makeYear(1916, [curatedEvent(), ...linkedLetters(1916, 1)]),
+			eventLookup: lookup
+		});
+		expect(document.querySelectorAll('[data-testid="event-card"]')).toHaveLength(1);
+		const titles = (document.body.textContent?.match(/Ein gewaltiger Stadtbrand/g) ?? []).length;
+		expect(titles).toBe(1);
+		// the letter is inside the card, not a loose .letter-row
+		expect(document.querySelector('a.lcard.ev')).not.toBeNull();
+		expect(document.querySelector('.letter-row')).toBeNull();
+		// no plain EventPill for it (the pill is the only floating .rounded-full wrapper)
+		expect(document.querySelector('.justify-center .rounded-full.border-brand-mint')).toBeNull();
+	});
+
+	it('renders a curated event with NO linked letters as a plain EventPill, no card (REQ-005)', () => {
+		render(YearBand, {
+			year: makeYear(1916, [curatedEvent()]),
+			eventLookup: lookup
+		});
+		expect(document.querySelector('[data-testid="event-card"]')).toBeNull();
+		// the curated EventPill is the bordered floating rounded-full wrapper
+		expect(
+			document.querySelector('.justify-center .rounded-full.border-brand-mint')
+		).not.toBeNull();
+		expect(document.body.textContent).toContain('Ein gewaltiger Stadtbrand');
+	});
+
+	it('renders loose (unlinked) letters in a sparse year as alternating .letter-row, no card (REQ-006)', () => {
+		const loose = manyLetters(1916, 3); // no linkedEventId
+		render(YearBand, { year: makeYear(1916, loose), eventLookup: lookup });
+		expect(document.querySelectorAll('.letter-row')).toHaveLength(3);
+		expect(document.querySelector('[data-testid="event-card"]')).toBeNull();
+	});
+
+	it('counts only loose letters in the density strip; event letters stay in the card (REQ-006/007)', () => {
+		// 15 loose letters fold into one strip; a 3-letter event card shows its 3.
+		const loose = manyLetters(1916, 15);
+		const year = makeYear(1916, [curatedEvent(), ...linkedLetters(1916, 3), ...loose]);
+		render(YearBand, { year, eventLookup: lookup });
+		// the event card holds 3 letters
+		expect(document.querySelectorAll('a.lcard.ev')).toHaveLength(3);
+		// the loose letters fold into exactly one density strip
+		const strips = document.querySelectorAll('[data-testid="strip-expand"]');
+		expect(strips).toHaveLength(1);
+		// the strip card's count text is 15 (the loose letters), not 18 (REQ-006/007)
+		const stripCard = strips[0].closest('.max-w-md') as HTMLElement;
+		expect(stripCard.textContent).toContain('15');
+		expect(stripCard.textContent).not.toContain('18');
+	});
+
+	it('renders a cross-year cluster (event not in this band) as a ✉ text-header card holding it (REQ-004)', () => {
+		// The event id is in eventLookup but no matching EVENT entry sits in this band.
+		render(YearBand, {
+			year: makeYear(1917, linkedLetters(1917, 2)),
+			eventLookup: lookup
+		});
+		const card = document.querySelector('[data-testid="event-card"]');
+		expect(card).not.toBeNull();
+		expect(document.body.textContent).toContain('✉');
+		expect(document.body.textContent).toContain('Ein gewaltiger Stadtbrand');
+		// cross-year card carries no edit link and no pill
+		expect(document.querySelector('[data-testid="event-edit"]')).toBeNull();
+		expect(document.querySelector('.justify-center .rounded-full.border-brand-mint')).toBeNull();
+	});
+
+	it('interleaves a cross-year card before a later-dated loose letter in the same band (REQ-015)', () => {
+		// Chronological band order (what the backend delivers): a February cross-year letter, then
+		// a November loose letter. The cross-year card must sit at its earliest letter's position —
+		// before the November loose letter — so the band still reads in strict time order.
+		const febLinked = makeEntry({
+			eventDate: '1917-02-10',
+			documentId: 'feb-linked',
+			title: 'Feldpostbrief',
+			linkedEventId: EV_ID
+		});
+		const novLoose = makeEntry({
+			eventDate: '1917-11-20',
+			documentId: 'nov-loose',
+			title: 'Brief im November'
+		});
+		render(YearBand, { year: makeYear(1917, [febLinked, novLoose]), eventLookup: lookup });
+		const card = document.querySelector('[data-testid="event-card"]') as HTMLElement;
+		const looseLink = document.querySelector('a[href="/documents/nov-loose"]') as HTMLElement;
+		expect(card).not.toBeNull();
+		expect(looseLink).not.toBeNull();
+		// the cross-year card precedes the later-dated loose letter in DOM order
+		expect(card.compareDocumentPosition(looseLink) & Node.DOCUMENT_POSITION_FOLLOWING).toBeTruthy();
+	});
+});

frontend/src/lib/timeline/eventCardConfig.spec.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { getAccentConfig } from './eventCardConfig';
+import { getAccentConfig, canEditEvent } from './eventCardConfig';
 import type { components } from '$lib/generated/api';
 
 type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
@@ -51,3 +51,24 @@ describe('getAccentConfig', () => {
 		expect(cfg.accent).toBe('curated');
 	});
 });
+
+// The single source of the curator edit-affordance gate (CLAUDE.md's TimelineEntryDTO contract):
+// a curated event shows its edit pencil only for a writer, never for a derived life-event or a
+// null eventId. Shared by EventPill, WorldBand, and EventCluster (#850 finding #5).
+describe('canEditEvent', () => {
+	it('allows a writer to edit a curated event with an eventId', () => {
+		expect(canEditEvent(event({ derived: false, eventId: 'e-1' }), true)).toBe(true);
+	});
+
+	it('denies a viewer without write permission', () => {
+		expect(canEditEvent(event({ derived: false, eventId: 'e-1' }), false)).toBe(false);
+	});
+
+	it('denies a derived life-event even for a writer', () => {
+		expect(canEditEvent(event({ derived: true, eventId: 'e-1' }), true)).toBe(false);
+	});
+
+	it('denies an event with no eventId even for a writer', () => {
+		expect(canEditEvent(event({ derived: false, eventId: undefined }), true)).toBe(false);
+	});
+});

frontend/src/lib/timeline/eventCardConfig.ts

@@ -36,3 +36,16 @@ export function getAccentConfig(entry: TimelineEntryDTO): AccentConfig {
 	}
 	return { glyph: '★', label: m.timeline_layer_family(), accent: 'curated' };
 }
+
+/**
+ * The curator edit-affordance gate, in one place — the security-relevant contract documented on
+ * CLAUDE.md's `TimelineEntryDTO` row (`derived || eventId == null` → no edit link). A curated
+ * event's edit pencil shows only for a viewer with WRITE_ALL (`canWrite`), and only when it is a
+ * real curated event: never a derived life-event (nothing to edit) and never a null `eventId`.
+ * HISTORICAL events are never derived, so this also covers the world band. The gate is UX only —
+ * the #781 route guard + backend permission are the real boundary. Shared by EventPill, WorldBand,
+ * and EventCluster so the gate has a single source of truth (#850 finding #5).
+ */
+export function canEditEvent(entry: TimelineEntryDTO, canWrite: boolean): boolean {
+	return canWrite && !entry.derived && entry.eventId != null;
+}

frontend/src/lib/timeline/eventClustering.spec.ts

@@ -0,0 +1,141 @@
+import { describe, it, expect } from 'vitest';
+import { buildEventLookup, splitYearLetters, CLUSTER_PREVIEW } from './eventClustering';
+import { makeEntry } from './test-factories';
+import type { components } from '$lib/generated/api';
+
+type TimelineDTO = components['schemas']['TimelineDTO'];
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+const EV_A = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa';
+const EV_B = 'bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb';
+
+const makeEvent = (overrides: Partial<TimelineEntryDTO> = {}): TimelineEntryDTO =>
+	makeEntry({ kind: 'EVENT', type: 'PERSONAL', documentId: undefined, ...overrides });
+
+describe('eventClustering — buildEventLookup', () => {
+	it('maps a curated year-band event to its title, excluding undated-bucket events (#7)', () => {
+		const timeline: TimelineDTO = {
+			years: [
+				{
+					year: 1916,
+					entries: [makeEvent({ eventId: EV_A, title: 'Ein gewaltiger Stadtbrand' })]
+				}
+			],
+			undated: [makeEvent({ eventId: EV_B, title: 'Briefe von der Front' })]
+		};
+		const lookup = buildEventLookup(timeline);
+		expect(lookup.get(EV_A)).toBe('Ein gewaltiger Stadtbrand');
+		// An undated event renders as a plain pill in the undated bucket — out of clustering
+		// scope. Including it here would scatter its dated letters into orphaned ✉ cross-year
+		// cards detached from the pill (#7), so it must NOT enter the lookup.
+		expect(lookup.has(EV_B)).toBe(false);
+		expect(lookup.size).toBe(1);
+	});
+
+	it('ignores derived events (no eventId) and letters', () => {
+		const timeline: TimelineDTO = {
+			years: [
+				{
+					year: 1916,
+					entries: [
+						makeEvent({ eventId: undefined, title: 'Geburt' }), // derived
+						makeEntry({ kind: 'LETTER', documentId: 'doc-1' })
+					]
+				}
+			],
+			undated: []
+		};
+		expect(buildEventLookup(timeline).size).toBe(0);
+	});
+
+	it('excludes a HISTORICAL event so its letters stay loose, keeping its WorldBand (REQ-014)', () => {
+		const timeline: TimelineDTO = {
+			years: [
+				{ year: 1916, entries: [makeEvent({ eventId: EV_A, type: 'HISTORICAL', title: 'Somme' })] }
+			],
+			undated: []
+		};
+		const lookup = buildEventLookup(timeline);
+		expect(lookup.has(EV_A)).toBe(false);
+		expect(lookup.size).toBe(0);
+	});
+
+	it('skips an event with an empty or whitespace title — no bare ✉ card (#8)', () => {
+		const timeline: TimelineDTO = {
+			years: [
+				{
+					year: 1916,
+					entries: [
+						makeEvent({ eventId: EV_A, title: '' }),
+						makeEvent({ eventId: EV_B, title: '   ' })
+					]
+				}
+			],
+			undated: []
+		};
+		expect(buildEventLookup(timeline).size).toBe(0);
+	});
+});
+
+describe('eventClustering — splitYearLetters', () => {
+	it('exposes a CLUSTER_PREVIEW of 5', () => {
+		expect(CLUSTER_PREVIEW).toBe(5);
+	});
+
+	it('clusters letters by linkedEventId with matching counts', () => {
+		const lookup = new Map([[EV_A, 'Stadtbrand']]);
+		const letters = [
+			makeEntry({ kind: 'LETTER', documentId: 'd1', linkedEventId: EV_A }),
+			makeEntry({ kind: 'LETTER', documentId: 'd2', linkedEventId: EV_A })
+		];
+		const { clusters, loose } = splitYearLetters(letters, lookup);
+		expect(clusters).toHaveLength(1);
+		expect(clusters[0].eventId).toBe(EV_A);
+		expect(clusters[0].title).toBe('Stadtbrand');
+		expect(clusters[0].letters).toHaveLength(2);
+		expect(loose).toHaveLength(0);
+	});
+
+	it('keeps a letter with no linkedEventId loose', () => {
+		const lookup = new Map([[EV_A, 'Stadtbrand']]);
+		const letters = [makeEntry({ kind: 'LETTER', documentId: 'd1', linkedEventId: undefined })];
+		const { clusters, loose } = splitYearLetters(letters, lookup);
+		expect(clusters).toHaveLength(0);
+		expect(loose).toHaveLength(1);
+	});
+
+	it('keeps a letter whose linkedEventId is absent from the lookup loose (filter-then-cluster, REQ-008)', () => {
+		const lookup = new Map([[EV_A, 'Stadtbrand']]);
+		const letters = [makeEntry({ kind: 'LETTER', documentId: 'd1', linkedEventId: EV_B })];
+		const { clusters, loose } = splitYearLetters(letters, lookup);
+		expect(clusters).toHaveLength(0);
+		expect(loose).toHaveLength(1);
+	});
+
+	it('places each letter in exactly one place (REQ-007)', () => {
+		const lookup = new Map([[EV_A, 'Stadtbrand']]);
+		const letters = [
+			makeEntry({ kind: 'LETTER', documentId: 'd1', linkedEventId: EV_A }),
+			makeEntry({ kind: 'LETTER', documentId: 'd2', linkedEventId: undefined }),
+			makeEntry({ kind: 'LETTER', documentId: 'd3', linkedEventId: EV_B })
+		];
+		const { clusters, loose } = splitYearLetters(letters, lookup);
+		const clustered = clusters.flatMap((c) => c.letters.length).reduce((a, b) => a + b, 0);
+		expect(clustered + loose.length).toBe(3);
+		expect(clustered).toBe(1);
+		expect(loose).toHaveLength(2);
+	});
+
+	it('keeps clusters in first-seen order', () => {
+		const lookup = new Map([
+			[EV_B, 'Front'],
+			[EV_A, 'Stadtbrand']
+		]);
+		const letters = [
+			makeEntry({ kind: 'LETTER', documentId: 'd1', linkedEventId: EV_A }),
+			makeEntry({ kind: 'LETTER', documentId: 'd2', linkedEventId: EV_B })
+		];
+		const { clusters } = splitYearLetters(letters, lookup);
+		expect(clusters.map((c) => c.eventId)).toEqual([EV_A, EV_B]);
+	});
+});

frontend/src/lib/timeline/eventClustering.ts

@@ -0,0 +1,88 @@
+import type { components } from '$lib/generated/api';
+
+type TimelineDTO = components['schemas']['TimelineDTO'];
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+/** Letters shown inside an event card before a "show more" toggle appears (#850, REQ-003). */
+export const CLUSTER_PREVIEW = 5;
+
+/** One contained event card's worth of letters within a year band (#850). */
+export interface EventCluster {
+	/** The curated event's id — also the `{#each}` key. */
+	eventId: string;
+	/** The curated event's title (from the event lookup). */
+	title: string;
+	letters: TimelineEntryDTO[];
+}
+
+/** The result of splitting a year's letters into event clusters and the loose remainder. */
+export interface SplitLetters {
+	clusters: EventCluster[];
+	loose: TimelineEntryDTO[];
+	/** Clusters keyed by `eventId` for O(1) lookup during row assembly (a letter's disposition is
+	 *  `byEvent.has(linkedEventId)`; an event's card is `byEvent.get(eventId)`). */
+	byEvent: Map<string, EventCluster>;
+}
+
+/**
+ * Maps each curated event present in the (already layer-filtered) timeline to its title. These
+ * are the only events a letter may cluster under — a letter whose `linkedEventId` is absent here
+ * links to an event the #780 layer filter removed, so it falls back to a loose chronological
+ * letter (filter-then-cluster, REQ-008). Curated PERSONAL events carry an `eventId`; derived
+ * life-events and letters do not, so they never enter the lookup. HISTORICAL events are excluded
+ * too: a world event always keeps its full-width WorldBand and never clusters, even with linked
+ * letters (REQ-014) — those letters stay loose.
+ *
+ * Only year-band events are collected: an undated event renders as a plain pill in the undated
+ * bucket (out of clustering scope), so including it would scatter its dated letters into orphaned
+ * cross-year cards detached from that pill (#7).
+ *
+ * An event with an empty/whitespace title is skipped too — clustering under it would render a
+ * label-less `✉` mystery card; its letters stay loose instead (#8).
+ */
+export function buildEventLookup(timeline: TimelineDTO): Map<string, string> {
+	const lookup = new Map<string, string>();
+	const collect = (entries: TimelineEntryDTO[]) => {
+		for (const entry of entries) {
+			const title = entry.title?.trim();
+			if (entry.kind === 'EVENT' && entry.eventId && entry.type !== 'HISTORICAL' && title) {
+				lookup.set(entry.eventId, title);
+			}
+		}
+	};
+	for (const band of timeline.years) collect(band.entries);
+	return lookup;
+}
+
+/**
+ * Splits one year's `LETTER` entries into event clusters and the loose remainder. A letter joins
+ * the cluster keyed by its `linkedEventId` IFF that id is set AND present in `eventLookup`
+ * (filter-then-cluster, REQ-007/008); every other letter is loose and stays in the chronological
+ * flow (REQ-006). Clusters keep first-seen order; each letter appears in exactly one place.
+ */
+export function splitYearLetters(
+	letters: TimelineEntryDTO[],
+	eventLookup?: Map<string, string>
+): SplitLetters {
+	const byEvent = new Map<string, EventCluster>();
+	const clusters: EventCluster[] = [];
+	const loose: TimelineEntryDTO[] = [];
+
+	for (const letter of letters) {
+		const eventId = letter.linkedEventId;
+		const title = eventId != null ? eventLookup?.get(eventId) : undefined;
+		if (eventId != null && title !== undefined) {
+			let cluster = byEvent.get(eventId);
+			if (!cluster) {
+				cluster = { eventId, title, letters: [] };
+				byEvent.set(eventId, cluster);
+				clusters.push(cluster);
+			}
+			cluster.letters.push(letter);
+		} else {
+			loose.push(letter);
+		}
+	}
+
+	return { clusters, loose, byEvent };
+}

frontend/src/lib/timeline/timeline-no-raw-html.spec.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { readdirSync, readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+
+const timelineDir = dirname(fileURLToPath(import.meta.url));
+
+/**
+ * REQ-010 / CWE-79: inline event clustering renders curator event titles and import-derived
+ * letter titles + sender/receiver text through every component under lib/timeline (the reused
+ * LetterCard, the new EventCluster card, the existing pills/bands/strip). That text must always
+ * render through Svelte's default `{...}` escaping — never `{@html}`. This grep gate fails loudly
+ * the moment any timeline component reaches for the raw-HTML directive.
+ */
+describe('lib/timeline never uses {@html} (REQ-010)', () => {
+	it('no timeline component contains the raw-HTML directive', () => {
+		const components = readdirSync(timelineDir).filter((file) => file.endsWith('.svelte'));
+		expect(components.length).toBeGreaterThan(0);
+		const offenders = components.filter((file) =>
+			readFileSync(join(timelineDir, file), 'utf8').includes('{@html')
+		);
+		expect(offenders).toEqual([]);
+	});
+});

frontend/src/routes/zeitstrahl/+page.svelte

@@ -60,7 +60,7 @@ const metaLine = $derived.by(() => {
 				: m.timeline_events_count({ count: meta.eventCount })
 		);
 	}
-	segments.push(m.timeline_grouping_date());
+	// REQ-011: the toggle-free chronological view carries no grouping segment.
 	return segments.join(' · ');
 });
 </script>

frontend/src/routes/zeitstrahl/page.svelte.spec.ts

@@ -43,7 +43,7 @@ describe('/zeitstrahl page', () => {
 		expect(canvas?.querySelector('ol')).not.toBeNull();
 	});
 
-	it('renders the meta sub-line with range, counts, and grouping (REQ-002)', () => {
+	it('renders the meta sub-line with range and counts, no grouping segment (REQ-011)', () => {
 		const dto = makeTimelineDTO({
 			years: [
 				makeYear(1909, [
@@ -59,7 +59,8 @@ describe('/zeitstrahl page', () => {
 		expect(sub?.textContent).toContain('1909–1924');
 		expect(sub?.textContent).toContain(m.timeline_letters_count({ count: 3 }));
 		expect(sub?.textContent).toContain(m.timeline_events_count({ count: 2 }));
-		expect(sub?.textContent).toContain(m.timeline_grouping_date());
+		// REQ-011: the toggle-free view drops the grouping meta segment.
+		expect(sub?.textContent).not.toContain('Gruppierung');
 	});
 
 	it('omits the range segment when there are no year bands (REQ-002)', () => {
@@ -84,7 +85,7 @@ describe('/zeitstrahl page', () => {
 		const sub = document.querySelector('[data-testid="timeline-meta"]');
 		expect(sub).not.toBeNull();
 		expect(sub?.textContent).not.toContain(m.timeline_letters_count({ count: 0 }));
-		expect(sub?.textContent).toContain(m.timeline_grouping_date());
+		expect(sub?.textContent).not.toContain('Gruppierung');
 	});
 
 	it('drops the events segment instead of showing "0 Ereignisse" (REQ-002)', () => {