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/prefetcher.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../likes/likes_provider.dart' show likedIdsProvider;
 import '../player/album_color_extractor.dart';
 import '../player/player_provider.dart';
 import 'audio_cache_manager.dart';
@@ -86,10 +87,17 @@ class Prefetcher {
       }
     }
 
-    // Eviction pass after pinning new files.
-    if (settings.capBytes > 0) {
-      await mgr.evict(targetBytes: settings.capBytes);
-    }
+    // Eviction pass after pinning new files. Per-bucket (#427 S2):
+    // liked-ness decides the bucket, so a track that's both liked
+    // and recently played is protected by the Liked cap, not the
+    // rolling LRU.
+    final liked =
+        _ref.read(likedIdsProvider).value?.tracks ?? const <String>{};
+    await mgr.evictBuckets(
+      likedCap: settings.likedCapBytes,
+      rollingCap: settings.rollingCapBytes,
+      liked: liked,
+    );
   }
 }