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/routes/documents/+page.ts

@@ -1,10 +1,23 @@
 import { browser } from '$app/environment';
-import { fetchDensity } from '$lib/document/timeline';
+import { fetchDensity, type DensityFilters } from '$lib/document/timeline';
 import type { PageLoad } from './$types';
 
 export const load: PageLoad = async ({ url, fetch, data }) => {
 	const view = url.searchParams.get('view');
 	const isDesktop = browser && window.matchMedia('(min-width: 640px)').matches;
-	const density = await fetchDensity(fetch, view, isDesktop);
+
+	// Forward active filters (excluding from/to) so the chart matches the list.
+	const tagOp = url.searchParams.get('tagOp');
+	const filters: DensityFilters = {
+		q: url.searchParams.get('q') ?? undefined,
+		senderId: url.searchParams.get('senderId') ?? undefined,
+		receiverId: url.searchParams.get('receiverId') ?? undefined,
+		tags: url.searchParams.getAll('tag'),
+		tagQ: url.searchParams.get('tagQ') ?? undefined,
+		status: url.searchParams.get('status') ?? undefined,
+		tagOp: tagOp === 'OR' ? 'OR' : 'AND'
+	};
+
+	const density = await fetchDensity(fetch, view, isDesktop, filters);
 	return { ...data, ...density };
 };