fix(flutter): analyze errors after Plan B push

Cleared 5 errors + 1 warning + 1 info from CI flutter analyze on 5114a81:

- prefetcher.dart: Riverpod listener callbacks used (_, _) for two
  placeholder params — Dart 3 enforces unique names. Changed to (_, __).
- prefetcher.dart: Riverpod 3's AsyncValue exposes `.value` (nullable)
  not `.valueOrNull`. Three call sites updated.
- sync_controller.dart: doc comment had `?since=<cursor>` → analyzer
  warned about unintended HTML. Wrapped in backticks with `{cursor}`.
- sync_controller.dart: delete loop over heterogeneous Table list
  inferred as List<Table>; drift's delete() expects TableInfo. Unrolled
  to explicit per-table deletes.
- audio_cache_manager_test.dart: db.into(...).insertAll([...]) doesn't
  exist on InsertStatement — only insert/insertOnConflictUpdate. Used
  db.batch((b) => b.insertAll(table, [...])) instead, in two test cases.
- audio_handler.dart: LockCachingAudioSource is marked experimental in
  just_audio. Added // ignore: experimental_member_use — operator
  acknowledged the experimental status during brainstorming and we're
  the project; CI's --fatal-infos would otherwise gate.

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

flutter_client/lib/cache/prefetcher.dart

@@ -18,18 +18,18 @@ class Prefetcher {
     // The mediaItem stream changes whenever the active track changes
     // (skipNext/skipPrev or natural progression). Settings changes (e.g.
     // operator bumps prefetch window) also trigger a reconcile.
-    _ref.listen<AsyncValue<MediaItem?>>(mediaItemProvider, (_, _) => _reconcile());
-    _ref.listen<AsyncValue<CacheSettings>>(cacheSettingsProvider, (_, _) => _reconcile());
+    _ref.listen<AsyncValue<MediaItem?>>(mediaItemProvider, (_, __) => _reconcile());
+    _ref.listen<AsyncValue<CacheSettings>>(cacheSettingsProvider, (_, __) => _reconcile());
   }
 
   final Ref _ref;
 
   Future<void> _reconcile() async {
-    final settings = _ref.read(cacheSettingsProvider).valueOrNull;
+    final settings = _ref.read(cacheSettingsProvider).value;
     if (settings == null) return;
 
-    final queue = _ref.read(queueProvider).valueOrNull ?? const <MediaItem>[];
-    final current = _ref.read(mediaItemProvider).valueOrNull;
+    final queue = _ref.read(queueProvider).value ?? const <MediaItem>[];
+    final current = _ref.read(mediaItemProvider).value;
     if (queue.isEmpty || current == null) return;
 
     final currentIdx = queue.indexWhere((m) => m.id == current.id);