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/player/audio_handler.dart

@@ -114,6 +114,7 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
       final cacheFile = File('${cacheDir.path}/audio_cache/${t.id}.mp3');
       debugPrint('audio_handler: cache miss for track.id=${t.id}, '
           'using LockCachingAudioSource → ${cacheFile.path}');
+      // ignore: experimental_member_use
       return LockCachingAudioSource(parsed,
           headers: headers, cacheFile: cacheFile);
     }