Add Library Cleanup admin page — missing files, orphaned records, stuck jobs, duplicates

New read-only scan (GET /api/library/cleanup/scan) across movies, episodes,
tracks, subtitles, HLS output, transcode queue, and watchlist/progress rows:
- Missing files: DB record exists but the file is gone from disk
- Orphaned episodes: show_id no longer exists
- Orphaned HLS output: transcoded folders left behind by deleted content
- Stuck transcode jobs: "running" 2+ hours with no completion (backend-restart artifact)
- Duplicate imports: same file imported into the library more than once
- Orphaned subtitles / dangling watchlist+progress rows

Nothing is deleted until an admin clicks Fix (per-item or Fix All per
category) via POST /api/library/cleanup/fix.

frontend/src/App.js

@@ -28,6 +28,7 @@ import Users from "./pages/Users";
 import Music from "./pages/Music";
 import AlbumDetail from "./pages/AlbumDetail";
 import LibraryScan from "./pages/LibraryScan";
+import LibraryCleanup from "./pages/LibraryCleanup";
 
 const ProfileGate = ({ children }) => {
   const { user, loading: authLoading } = useAuth();
@@ -101,6 +102,7 @@ function App() {
                   <Route path="/admin/radarr" element={<AdminGate><RadarrImport /></AdminGate>} />
                   <Route path="/admin/sonarr" element={<AdminGate><SonarrImport /></AdminGate>} />
                   <Route path="/admin/scan" element={<AdminGate><LibraryScan /></AdminGate>} />
+                  <Route path="/admin/cleanup" element={<AdminGate><LibraryCleanup /></AdminGate>} />
                   <Route path="/admin/shows/:id/episodes" element={<AdminGate><AdminShowEpisodes /></AdminGate>} />
                   <Route path="/admin/queue" element={<AdminGate><TranscodeQueue /></AdminGate>} />
                   <Route path="*" element={<Navigate to="/" replace />} />