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/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",