feat(timeline): thread event clustering through TimelineView; drop the grouping meta segment

TimelineView builds the event lookup once over the whole timeline and threads
it (plus canWrite) to every YearBand, so a curated event's letters cluster
under it inline. The /zeitstrahl meta-line drops its 'Gruppierung: Datum'
segment (toggle-free view, REQ-011); the now-unused timeline_grouping_date
key is removed from de/en/es and the messages parity guard, which now asserts
the new show-more/less keys.

Refs #850

frontend/src/lib/messages.spec.ts

@@ -74,9 +74,10 @@ describe('message key parity', () => {
 	// every locale so no surface ever falls back to a missing translation.
 	it('zeitstrahl visual-fidelity keys are present in all locales (#833 REQ-015)', () => {
 		const requiredKeys = [
-			'timeline_grouping_date',
 			'timeline_provenance_derived',
 			'timeline_provenance_curated',
+			'timeline_bucket_show_more',
+			'timeline_bucket_show_less',
 			'timeline_letter_glyph_label',
 			'timeline_layer_historical_suffix',
 			'timeline_strip_density_caption',

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}

frontend/src/lib/timeline/TimelineView.svelte.spec.ts

@@ -341,4 +341,34 @@ describe('TimelineView', () => {
 		expect(hrefs).toContain('/zeitstrahl/events/wb/edit');
 		expect(hrefs).toContain('/zeitstrahl/events/wu/edit');
 	});
+
+	it('builds the event lookup and clusters a curated event + same-year linked letter into an event-card (#850)', () => {
+		const evId = 'eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee';
+		const event = makeEntry({
+			kind: 'EVENT',
+			type: 'PERSONAL',
+			derived: false,
+			eventId: evId,
+			eventDate: '1916-07-06',
+			precision: 'DAY',
+			title: 'Ein gewaltiger Stadtbrand',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined
+		});
+		const letter = makeEntry({
+			eventDate: '1916-05-10',
+			documentId: 'doc-linked',
+			title: 'Brief',
+			linkedEventId: evId
+		});
+		render(TimelineView, {
+			timeline: makeTimelineDTO({ years: [makeYear(1916, [event, letter])] })
+		});
+		expect(document.querySelector('[data-testid="event-card"]')).not.toBeNull();
+		expect(document.querySelector('a.lcard.ev')).not.toBeNull();
+		// the title reads once — the event is the card header, not also a loose pill
+		const titles = (document.body.textContent?.match(/Ein gewaltiger Stadtbrand/g) ?? []).length;
+		expect(titles).toBe(1);
+	});
 });