Library search upgrades: tag search, tag cloud, status filter

Backend:
- MediaItem gains search_tags (custom + e621 tags, lowercase) and
  has_custom_data, maintained on save with a data migration backfill
- File list search accepts search_type=filename|tags|both (tag search is
  word-AND across the flattened tag text) and status=matched|custom|
  unknown filters
- New /api/tags/cloud/ endpoint (cached 2 min per guest/auth, invalidated
  on item changes and deletions) returning the most-used tags, honouring
  guest visibility

Frontend:
- Library sidebar: Filename/Tags/Both selector, status pill toggles
  (persisted), and a clickable tag cloud that runs a tag search
- Roadmap updated

backend/apps/library/urls.py

@@ -6,6 +6,7 @@ from .tools import (
     DeleteFilesView,
     ExactDuplicatesView,
     StorageView,
+    TagCloudView,
     VisualGroupsView,
     VisualMatchesView,
 )
@@ -37,5 +38,6 @@ urlpatterns = [
     ),
     path("delete/", DeleteFilesView.as_view(), name="delete_files"),
     path("temp/clear/", ClearTempView.as_view(), name="clear_temp"),
+    path("tags/cloud/", TagCloudView.as_view(), name="tag_cloud"),
     path("storage/", StorageView.as_view(), name="storage_info"),
 ]