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/sync_controller.dart

@@ -19,7 +19,7 @@ class SyncResult {
 }
 
 /// Drives the delta-sync against the server (#357 Plan A endpoint).
-/// Reads cursor from drift, calls /api/library/sync?since=<cursor>,
+/// Reads cursor from drift, calls `/api/library/sync?since={cursor}`,
 /// applies upserts + deletes, advances cursor.
 class SyncController extends AsyncNotifier<SyncResult?> {
   @override
@@ -55,16 +55,12 @@ class SyncController extends AsyncNotifier<SyncResult?> {
       // local state and retry once with cursor=0.
       if (resp.statusCode == 410) {
         await db.transaction(() async {
-          for (final t in [
-            db.cachedArtists,
-            db.cachedAlbums,
-            db.cachedTracks,
-            db.cachedLikes,
-            db.cachedPlaylists,
-            db.cachedPlaylistTracks,
-          ]) {
-            await db.delete(t).go();
-          }
+          await db.delete(db.cachedArtists).go();
+          await db.delete(db.cachedAlbums).go();
+          await db.delete(db.cachedTracks).go();
+          await db.delete(db.cachedLikes).go();
+          await db.delete(db.cachedPlaylists).go();
+          await db.delete(db.cachedPlaylistTracks).go();
           await db.into(db.syncMetadata).insertOnConflictUpdate(
                 SyncMetadataCompanion.insert(cursor: const drift.Value(0)),
               );