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/generated/api.ts

@@ -2253,14 +2253,14 @@ export interface components {
             /** Format: int32 */
             totalPages?: number;
             pageable?: components["schemas"]["PageableObject"];
+            first?: boolean;
+            last?: boolean;
             /** Format: int32 */
             size?: number;
             content?: components["schemas"]["NotificationDTO"][];
             /** Format: int32 */
             number?: number;
             sort?: components["schemas"]["SortObject"];
-            first?: boolean;
-            last?: boolean;
             /** Format: int32 */
             numberOfElements?: number;
             empty?: boolean;
@@ -4959,8 +4959,15 @@ export interface operations {
     density: {
         parameters: {
             query?: {
-                from?: string;
-                to?: string;
+                q?: string;
+                senderId?: string;
+                receiverId?: string;
+                tag?: string[];
+                tagQ?: string;
+                /** @description Filter by document status */
+                status?: "PLACEHOLDER" | "UPLOADED" | "TRANSCRIBED" | "REVIEWED" | "ARCHIVED";
+                /** @description Tag operator: AND (default) or OR */
+                tagOp?: string;
             };
             header?: never;
             path?: never;