feat(flutter/cache): drift schema + new deps + CI codegen step (#357 plan B)

Adds the offline-mode foundation:
- pubspec deps: drift, drift_flutter, sqlite3_flutter_libs,
  connectivity_plus + drift_dev/build_runner (dev)
- lib/cache/db.dart with 8 tables: CachedArtists/Albums/Tracks/Likes/
  Playlists/PlaylistTracks + AudioCacheIndex + SyncMetadata
- *.g.dart added to .gitignore (build_runner regenerates per build)
- CI workflow gains a 'Codegen (drift)' step between pub get and
  analyze so the generated symbols exist when analyze runs

CacheSource enum drives tiered LRU eviction (manual/autoLiked/
autoPlaylist/autoPrefetch/incidental). Subsequent commits add the
sync controller, audio cache manager, prefetcher, settings UI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.forgejo/workflows/flutter.yml

@@ -56,6 +56,12 @@ jobs:
       - name: Pub get
         run: flutter pub get
 
+      - name: Codegen (drift, etc.)
+        # build_runner generates *.g.dart files (drift schemas, etc.)
+        # which are gitignored. Must run before analyze + test so the
+        # generated symbols exist.
+        run: dart run build_runner build --delete-conflicting-outputs
+
       - name: Analyze
         run: flutter analyze --fatal-infos