feat(flutter/player): audio_handler reads from cache first; LockCaching fallback

_buildAudioSource is now async and cache-aware:
1. Cache hit  → AudioSource.uri(file://path)
2. Cache miss with manager → LockCachingAudioSource (cache-as-you-play)
3. No manager configured → plain AudioSource.uri (legacy fallback)

playerActionsProvider.playTracks now passes audioCacheManager into
configure() alongside coverCache. setQueueFromTracks awaits the source
build (Future.wait over the track list).

Out of scope: registering an index row when LockCachingAudioSource
finishes downloading (no clean hook from just_audio). Prefetcher /
Download buttons cover the index path.

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

flutter_client/lib/player/player_provider.dart

@@ -2,6 +2,7 @@ import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../auth/auth_provider.dart';
+import '../cache/audio_cache_manager.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/track.dart';
 import 'album_cover_cache.dart';
@@ -37,8 +38,14 @@ class PlayerActions {
     final url   = await _ref.read(serverUrlProvider.future);
     final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
     final cache = _ref.read(albumCoverCacheProvider);
+    final audioCache = _ref.read(audioCacheManagerProvider);
     final h = _ref.read(audioHandlerProvider)
-      ..configure(baseUrl: url ?? '', token: token, coverCache: cache);
+      ..configure(
+        baseUrl: url ?? '',
+        token: token,
+        coverCache: cache,
+        audioCacheManager: audioCache,
+      );
     await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
     await h.play();
   }