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/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;
+}