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/library/album_detail_screen.dart

@@ -3,8 +3,6 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/likes.dart';
 import '../models/album.dart';
-import '../cache/audio_cache_manager.dart';
-import '../cache/db.dart';
 import '../likes/like_button.dart';
 import '../player/player_provider.dart';
 import '../shared/widgets/server_image.dart';
@@ -113,21 +111,6 @@ class AlbumDetailScreen extends ConsumerWidget {
                       Text(r.album.artistName, style: TextStyle(color: fs.ash)),
                     ],
                   )),
-                  IconButton(
-                    key: const Key('download_album_button'),
-                    icon: Icon(Icons.download, color: fs.ash),
-                    tooltip: 'Download album',
-                    onPressed: () {
-                      final mgr = ref.read(audioCacheManagerProvider);
-                      for (final t in r.tracks) {
-                        // ignore: unawaited_futures
-                        mgr.pin(t.id, source: CacheSource.autoPlaylist);
-                      }
-                      ScaffoldMessenger.of(context).showSnackBar(
-                        SnackBar(content: Text('Downloading ${r.tracks.length} tracks…')),
-                      );
-                    },
-                  ),
                   Container(
                     width: 48, height: 48,
                     decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),