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

frontend/src/store/ui.ts

@@ -7,6 +7,8 @@ interface UiState {
   toggleSidebar: () => void;
   ratings: string[];
   setRatings: (ratings: string[]) => void;
+  statuses: string[];
+  setStatuses: (statuses: string[]) => void;
   perPage: number;
   setPerPage: (perPage: number) => void;
   ordering: string;
@@ -23,6 +25,9 @@ export const useUi = create<UiState>()(
       ratings: [],
       setRatings: (ratings) => set({ ratings }),
 
+      statuses: [],
+      setStatuses: (statuses) => set({ statuses }),
+
       perPage: 48,
       setPerPage: (perPage) => set({ perPage }),