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/test/cache/audio_cache_manager_test.dart

@@ -4,6 +4,7 @@ library;
 import 'dart:io';
 
 import 'package:dio/dio.dart';
+import 'package:drift/drift.dart' show Value;
 import 'package:drift/native.dart' show NativeDatabase;
 import 'package:flutter_test/flutter_test.dart';
 
@@ -54,7 +55,8 @@ void main() {
     expect(await mgr.usageBytes(), 350);
   });
 
-  test('eviction order: incidental first, manual never', skip: _skipDrift, () async {
+  test('rolling cap evicts non-liked LRU; liked protected',
+      skip: _skipDrift, () async {
     final db = _testDb();
     addTearDown(db.close);
     final tmp = Directory.systemTemp.createTempSync();
@@ -63,30 +65,47 @@ void main() {
       dioFactory: () async => Dio(),
       cacheDirFactory: () async => tmp,
     );
-    final f1 = File('${tmp.path}/audio_cache/inc.mp3');
-    await f1.create(recursive: true);
-    await f1.writeAsBytes(List.filled(100, 0));
-    final f2 = File('${tmp.path}/audio_cache/man.mp3');
-    await f2.create(recursive: true);
-    await f2.writeAsBytes(List.filled(100, 0));
+    Future<void> mk(String id) async {
+      final f = File('${tmp.path}/audio_cache/$id.mp3');
+      await f.create(recursive: true);
+      await f.writeAsBytes(List.filled(100, 0));
+    }
+    await mk('old');
+    await mk('new');
+    await mk('lik');
     await db.batch((b) {
       b.insertAll(db.audioCacheIndex, [
+        // 'old' has the older lastPlayedAt → evicted first.
         AudioCacheIndexCompanion.insert(
-            trackId: 'inc',
-            path: f1.path,
+            trackId: 'old',
+            path: '${tmp.path}/audio_cache/old.mp3',
             sizeBytes: 100,
-            source: CacheSource.incidental),
+            source: CacheSource.incidental,
+            lastPlayedAt: Value(DateTime(2020))),
         AudioCacheIndexCompanion.insert(
-            trackId: 'man',
-            path: f2.path,
+            trackId: 'new',
+            path: '${tmp.path}/audio_cache/new.mp3',
             sizeBytes: 100,
-            source: CacheSource.manual),
+            source: CacheSource.incidental,
+            lastPlayedAt: Value(DateTime(2024))),
+        AudioCacheIndexCompanion.insert(
+            trackId: 'lik',
+            path: '${tmp.path}/audio_cache/lik.mp3',
+            sizeBytes: 100,
+            source: CacheSource.incidental,
+            lastPlayedAt: Value(DateTime(2019))),
       ]);
     });
-    expect(await mgr.usageBytes(), 200);
-    await mgr.evict(targetBytes: 100);
-    expect(await mgr.isCached('inc'), false);
-    expect(await mgr.isCached('man'), true);
+    final liked = {'lik'};
+    final u = await mgr.bucketUsage(liked);
+    expect(u.liked, 100);
+    expect(u.rolling, 200);
+    // Rolling cap fits one 100-byte file; Liked cap huge.
+    await mgr.evictBuckets(
+        likedCap: 1 << 30, rollingCap: 100, liked: liked);
+    expect(await mgr.isCached('old'), false); // oldest rolling, evicted
+    expect(await mgr.isCached('new'), true); // newer rolling, kept
+    expect(await mgr.isCached('lik'), true); // liked, protected
   });
 
   test('clearAll removes everything including manual', skip: _skipDrift, () async {

flutter_client/test/cache/cache_settings_provider_test.dart

@@ -27,7 +27,9 @@ void main() {
     addTearDown(container.dispose);
 
     final s = await container.read(cacheSettingsProvider.future);
-    expect(s.capBytes, CacheSettings.defaults.capBytes);
+    expect(s.likedCapBytes, CacheSettings.defaults.likedCapBytes);
+    expect(s.rollingCapBytes, CacheSettings.defaults.rollingCapBytes);
+    expect(s.likedCapBytes, 5 * 1024 * 1024 * 1024);
     expect(s.prefetchWindow, 5);
     expect(s.cacheLikedTracks, true);
   });