feat(documents): timeline density refetches when other filters change (#385)

The +page.ts client-side load now forwards the active /documents URL
filters (q, senderId, receiverId, tag, tagQ, status, tagOp) to
/api/documents/density so the bars recompute when the user narrows the
search. Date bounds (from/to) are deliberately omitted — the chart is
the surface for picking those.

- New `DensityFilters` type and `buildDensityUrl(filters)` helper.
- `fetchDensity` accepts a filter snapshot (defaulting to {} for
  back-compat in tests).
- 6 new unit tests cover URL building, multi-tag repetition, AND/OR
  forwarding, the explicit-no-from/to invariant, and filter-aware fetch.
- Generated API types refreshed against the new backend signature.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/timeline.spec.ts

@@ -5,6 +5,7 @@ import {
 	buildMonthSequence,
 	fillDensityGaps,
 	fetchDensity,
+	buildDensityUrl,
 	aggregateToYears,
 	selectionBoundaryFrom,
 	selectionBoundaryTo
@@ -152,6 +153,39 @@ describe('selectionBoundaryFrom / To', () => {
 	});
 });
 
+describe('buildDensityUrl', () => {
+	it('returns the bare endpoint when no filters provided', () => {
+		expect(buildDensityUrl()).toBe('/api/documents/density');
+	});
+
+	it('forwards single-value filters as query params', () => {
+		expect(buildDensityUrl({ q: 'Brief', senderId: 's-1' })).toBe(
+			'/api/documents/density?q=Brief&senderId=s-1'
+		);
+	});
+
+	it('repeats the tag param for multi-value tag filters', () => {
+		const url = buildDensityUrl({ tags: ['Familie', 'Urlaub'], tagOp: 'OR' });
+		expect(url).toContain('tag=Familie');
+		expect(url).toContain('tag=Urlaub');
+		expect(url).toContain('tagOp=OR');
+	});
+
+	it('omits tagOp when it is AND (default on backend)', () => {
+		const url = buildDensityUrl({ tags: ['Familie'], tagOp: 'AND' });
+		expect(url).not.toContain('tagOp=');
+	});
+
+	it('does not forward from/to even if a caller mistakenly adds them', () => {
+		// Intentional: density is the surface for picking from/to, so it must always
+		// span the broader space the user is selecting within.
+		// @ts-expect-error - from/to are explicitly absent from DensityFilters
+		const url = buildDensityUrl({ q: 'Brief', from: '1915-01-01', to: '1916-12-31' });
+		expect(url).not.toContain('from=');
+		expect(url).not.toContain('to=');
+	});
+});
+
 describe('fetchDensity', () => {
 	it('skips fetch and returns null density on mobile', async () => {
 		const fetch = vi.fn();
@@ -189,6 +223,19 @@ describe('fetchDensity', () => {
 		expect(result.maxDate).toBe('1916-12-31');
 	});
 
+	it('forwards active filters as query params', async () => {
+		const fetch = vi.fn().mockResolvedValue({
+			ok: true,
+			json: async () => ({ buckets: [], minDate: null, maxDate: null })
+		});
+
+		await fetchDensity(fetch, null, true, { senderId: 's-1', tags: ['Familie'] });
+
+		const calledWith = fetch.mock.calls[0][0] as string;
+		expect(calledWith).toContain('senderId=s-1');
+		expect(calledWith).toContain('tag=Familie');
+	});
+
 	it('returns empty density and null bounds when the API responds non-ok', async () => {
 		const fetch = vi.fn().mockResolvedValue({ ok: false, status: 500 });