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/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}