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/playlists/playlist_detail_screen.dart

@@ -2,8 +2,6 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
-import '../cache/audio_cache_manager.dart';
-import '../cache/db.dart';
 import '../models/playlist.dart';
 import '../models/track.dart';
 import '../player/player_provider.dart';
@@ -266,33 +264,15 @@ class _Header extends ConsumerWidget {
           ),
           const Spacer(),
           if (playable.isNotEmpty) ...[
-            if (p.refreshable)
+            if (p.refreshable) ...[
               OutlinedButton.icon(
                 key: const Key('regenerate_playlist_button'),
                 onPressed: () => _regenerate(context, ref),
                 icon: const Icon(Icons.refresh, size: 16),
                 label: const Text('Regenerate'),
-              )
-            else
-              OutlinedButton.icon(
-                key: const Key('download_playlist_button'),
-                onPressed: () {
-                  final mgr = ref.read(audioCacheManagerProvider);
-                  for (final t in playable) {
-                    final id = t.trackId ?? '';
-                    if (id.isEmpty) continue;
-                    // Fire-and-forget; downloads happen in background.
-                    // ignore: unawaited_futures
-                    mgr.pin(id, source: CacheSource.autoPlaylist);
-                  }
-                  ScaffoldMessenger.of(context).showSnackBar(
-                    SnackBar(content: Text('Downloading ${playable.length} tracks…')),
-                  );
-                },
-                icon: const Icon(Icons.download, size: 16),
-                label: const Text('Download'),
               ),
-            const SizedBox(width: 8),
+              const SizedBox(width: 8),
+            ],
             FilledButton.icon(
               onPressed: () {
                 final refs = playable.map(_toTrackRef).toList(growable: false);