feat(offline): #427 S2(+S3) — two-bucket cache model

Replaces the single 5GB capBytes with independent Liked + Rolling
budgets (5GB each default). Bucket = liked-ness, NOT CacheSource:
a cached track currently in the liked set is charged to / evicted
under Liked; everything else is Rolling. Storage-only dedup — it
never filters playback (S4's offline lists query the whole index).

- db.dart: schema 8→9, AudioCacheIndex.lastPlayedAt (real play
  recency for S4 + rolling LRU; migration backfills to cachedAt).
  drift codegen run.
- cache_settings: likedCapBytes + rollingCapBytes (+ setters); old
  cache_cap_bytes key dropped, defaults reapply (not data loss).
- audio_cache_manager: touch(); bucketUsage() counts orphan
  partials (LockCaching files never indexed) as Rolling so the cap
  truly bounds disk; evictBuckets() drains non-liked LRU then
  sweeps orphans, Liked only by its own (large) cap — normal use
  never evicts the user's liked library.
- prefetcher → evictBuckets with the cached liked set.
- storage_section: two cap selectors + per-bucket usage (folds in
  S3 to avoid a broken intermediate).
- Explicit Download dropped: removed album + playlist Download
  buttons, autoPlaylist pins, now-unused imports.
- Tests updated/compiled (drift-cohort tests are CI-skipped).

High blast radius (eviction deletes files) — liked-protective by
design; needs operator device-check before "done".

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

flutter_client/lib/cache/db.dart

@@ -101,6 +101,11 @@ class AudioCacheIndex extends Table {
   TextColumn get path => text()();
   IntColumn get sizeBytes => integer()();
   DateTimeColumn get cachedAt => dateTime().withDefault(currentDateAndTime)();
+  /// When the track was last played. Drives the offline "Recently
+  /// played" ordering and rolling-bucket LRU eviction. Distinct from
+  /// cachedAt (download time). Nullable for pre-schema-9 rows until
+  /// the next play touches them (migration backfills to cachedAt).
+  DateTimeColumn get lastPlayedAt => dateTime().nullable()();
   TextColumn get source => textEnum<CacheSource>()();
   @override
   Set<Column> get primaryKey => {trackId};
@@ -247,7 +252,7 @@ class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 8;
+  int get schemaVersion => 9;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -300,6 +305,17 @@ class AppDb extends _$AppDb {
             // upgrade; populated by controllers when REST calls fail.
             await m.createTable(cachedMutations);
           }
+          if (from < 9) {
+            // Schema 9 (#427 S2): two-bucket cache. lastPlayedAt
+            // gives the offline "Recently played" view a real
+            // recency signal (cachedAt is download time, not play
+            // time). Nullable — backfilled to cachedAt so existing
+            // rows order sensibly until next play touches them.
+            await m.addColumn(audioCacheIndex, audioCacheIndex.lastPlayedAt);
+            await customStatement(
+              'UPDATE audio_cache_index SET last_played_at = cached_at',
+            );
+          }
         },
       );
 }