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

.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 |