docs(c4): document timeline density widget across backend+frontend (#385)

- l3-backend-3b: extend DocumentController description to include the
  per-month density aggregation endpoint.
- l3-frontend-3b: add /documents/+page.ts (client-side gated loader) and
  TimelineDensityFilter component, plus relationships to the density
  endpoint and the search dashboard.

Per Markus' follow-up §5: both diagrams are mandatory before merge.

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

docs/architecture/c4/l3-backend-3b-document-management.puml

@@ -8,7 +8,7 @@ ContainerDb(db, "PostgreSQL", "PostgreSQL 16")
 ContainerDb(minio, "Object Storage", "MinIO (S3-compatible)")
 
 System_Boundary(backend, "API Backend (Spring Boot)") {
-    Component(docCtrl, "DocumentController", "Spring MVC — /api/documents", "CRUD for documents: search, get by ID, update metadata, upload/download file, conversation thread, and batch metadata updates.")
+    Component(docCtrl, "DocumentController", "Spring MVC — /api/documents", "CRUD for documents: search, get by ID, update metadata, upload/download file, conversation thread, batch metadata updates, and per-month density aggregation for the timeline filter widget.")
     Component(adminCtrl, "AdminController", "Spring MVC — /api/admin", "Triggers asynchronous Excel/ODS mass import (requires ADMIN permission). Reports import state (IDLE/RUNNING/DONE/FAILED).")
     Component(docSvc, "DocumentService", "Spring Service", "Core document business logic: store, update, search. Resolves persons and tags, delegates file I/O to FileService, builds dynamic JPA Specifications, and integrates with audit logging.")
     Component(fileSvc, "FileService", "Spring Service", "Wraps AWS SDK v2 S3Client. Uploads files with UUID-keyed paths, computes SHA-256 hash, downloads with content-type detection, and generates presigned URLs for OCR access.")