Cluster event letters inline in the chronological /zeitstrahl (no grouping toggle) (#851)

Closes #850

## Summary

On `/zeitstrahl`, a curated event that has letters linked to it now renders as a contained event card — the event is the card header (accent glyph, title, `{date} · {kuratiert|abgeleitet}` subtitle, count, and a curator edit link), with its linked letters listed inside (first 5, then a keyboard-operable show-more/less toggle). Letters in a year *other* than the event's band get a lighter cross-year `✉ title` card. Every other letter stays a plain, alternating, density-folding chronological letter. There is **no grouping control** — clustering is automatic and always on. The meta-line drops its `Gruppierung: Datum` segment.

This supersedes #827: it keeps that branch's event-card clustering and the computed `linkedEventId`, and drops the toggle, the Thema mode, and the "Weitere Briefe" drawer.

## What changed

**Backend**
- `TimelineEntryDTO` gains a nullable `linkedEventId` (UUID; not `@Schema(REQUIRED)`).
- `TimelineService.resolveLetterEventLinks` resolves each letter's curated event in one batched pass over the events it already loads — no per-letter query, no new column, no Flyway migration.
- Regenerated the single `linkedEventId?` field in `api.ts`.

**Frontend**
- New `eventClustering.ts` (`buildEventLookup`, `splitYearLetters`, `CLUSTER_PREVIEW=5`) — filter-then-cluster: a letter clusters only if its `linkedEventId` is set AND present in the lookup, otherwise it stays loose.
- New `EventCluster.svelte` — the contained event card (same-year event header + edit link, or cross-year ✉ text header; first-5 + show-more).
- `LetterCard.svelte` gains `compact` + `variant='event'` (the `.lcard.ev` in-card letter).
- `YearBand.svelte` rebuilt to render event clusters inline; loose letters keep the alternating layout and density strip, and the strip counts **only** loose letters (no duplication).
- `TimelineView.svelte` builds the event lookup once and threads it + `canWrite` to each band.
- `+page.svelte` drops the grouping meta segment; the unused `timeline_grouping_date` key removed from de/en/es.
- New `timeline_bucket_show_more`/`_less` keys in all locales.
- REQ-010 `{@html}` grep gate over `lib/timeline/`.

## Tests (real runs)

- Backend `TimelineServiceTest`: **30 passed** (incl. the 2 new `linkedEventId` tests); `DerivedEventsAssemblyTest`: 17 passed; backend main sources compile.
- Frontend client sweep (`LetterCard`, `EventCluster`, `YearBand`, `TimelineView`, `zeitstrahl/page`): **81 passed** (5 files).
- Frontend server sweep (`eventClustering`, `messages`, `timeline-no-raw-html`): **18 passed** (3 files).
- `svelte-check`: no new errors in the touched files (pre-existing baseline noise elsewhere unchanged).

RTM: thirteen `REQ-001..013` rows added for #850 (feature `inline-event-clustering`), Status Done.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Marcel <marcel@familienarchiv>
Reviewed-on: #851

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>