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>

flutter_client/lib/cache/db.dart

@@ -0,0 +1,130 @@
+// Drift database for Minstrel's offline cache (#357 plan B).
+//
+// Two cache layers in one database:
+//   - Metadata cache (CachedArtists, CachedAlbums, CachedTracks,
+//     CachedLikes, CachedPlaylists, CachedPlaylistTracks) — populated
+//     by SyncController via /api/library/sync
+//   - Audio cache index (AudioCacheIndex) — owned by AudioCacheManager
+//
+// Plus SyncMetadata holding the latest sync cursor.
+//
+// All entity ids are TEXT (server-side UUIDs serialized as strings).
+// build_runner generates db.g.dart from this file; it's gitignored and
+// regenerated in CI.
+
+import 'package:drift/drift.dart';
+import 'package:drift_flutter/drift_flutter.dart';
+
+part 'db.g.dart';
+
+class CachedArtists extends Table {
+  TextColumn get id => text()();
+  TextColumn get name => text()();
+  TextColumn get sortName => text()();
+  TextColumn get mbid => text().nullable()();
+  TextColumn get artistThumbPath => text().nullable()();
+  TextColumn get artistFanartPath => text().nullable()();
+  DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
+class CachedAlbums extends Table {
+  TextColumn get id => text()();
+  TextColumn get artistId => text()();
+  TextColumn get title => text()();
+  TextColumn get sortTitle => text()();
+  TextColumn get releaseDate => text().nullable()();
+  TextColumn get coverPath => text().nullable()();
+  TextColumn get mbid => text().nullable()();
+  DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
+class CachedTracks extends Table {
+  TextColumn get id => text()();
+  TextColumn get albumId => text()();
+  TextColumn get artistId => text()();
+  TextColumn get title => text()();
+  IntColumn get durationMs => integer().withDefault(const Constant(0))();
+  IntColumn get trackNumber => integer().nullable()();
+  IntColumn get discNumber => integer().nullable()();
+  TextColumn get filePath => text().nullable()();
+  TextColumn get fileFormat => text().nullable()();
+  TextColumn get genre => text().nullable()();
+  DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
+class CachedLikes extends Table {
+  TextColumn get userId => text()();
+  TextColumn get entityType => text()(); // 'track' | 'album' | 'artist'
+  TextColumn get entityId => text()();
+  DateTimeColumn get likedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {userId, entityType, entityId};
+}
+
+class CachedPlaylists extends Table {
+  TextColumn get id => text()();
+  TextColumn get userId => text()();
+  TextColumn get name => text()();
+  TextColumn get description => text().withDefault(const Constant(''))();
+  BoolColumn get isPublic => boolean().withDefault(const Constant(false))();
+  TextColumn get coverPath => text().nullable()();
+  IntColumn get trackCount => integer().withDefault(const Constant(0))();
+  IntColumn get durationSec => integer().withDefault(const Constant(0))();
+  DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
+class CachedPlaylistTracks extends Table {
+  TextColumn get playlistId => text()();
+  TextColumn get trackId => text()();
+  IntColumn get position => integer().withDefault(const Constant(0))();
+  @override
+  Set<Column> get primaryKey => {playlistId, trackId};
+}
+
+/// One row per fully-downloaded audio file. `source` drives tiered LRU
+/// eviction; `incidental` evicts first, `manual` last.
+class AudioCacheIndex extends Table {
+  TextColumn get trackId => text()();
+  TextColumn get path => text()();
+  IntColumn get sizeBytes => integer()();
+  DateTimeColumn get cachedAt => dateTime().withDefault(currentDateAndTime)();
+  TextColumn get source => textEnum<CacheSource>()();
+  @override
+  Set<Column> get primaryKey => {trackId};
+}
+
+/// Single-row table holding the latest sync cursor.
+class SyncMetadata extends Table {
+  IntColumn get id => integer().withDefault(const Constant(1))();
+  IntColumn get cursor => integer().withDefault(const Constant(0))();
+  DateTimeColumn get lastSyncAt => dateTime().nullable()();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
+enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
+
+@DriftDatabase(tables: [
+  CachedArtists,
+  CachedAlbums,
+  CachedTracks,
+  CachedLikes,
+  CachedPlaylists,
+  CachedPlaylistTracks,
+  AudioCacheIndex,
+  SyncMetadata,
+])
+class AppDb extends _$AppDb {
+  AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
+
+  @override
+  int get schemaVersion => 1;
+}