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/test/cache/audio_cache_manager_test.dart

@@ -31,12 +31,20 @@ void main() {
       dioFactory: () async => Dio(),
       cacheDirFactory: () async => tmp,
     );
-    await db.into(db.audioCacheIndex).insertAll([
-      AudioCacheIndexCompanion.insert(
-          trackId: 'a', path: 'p', sizeBytes: 100, source: CacheSource.manual),
-      AudioCacheIndexCompanion.insert(
-          trackId: 'b', path: 'p', sizeBytes: 250, source: CacheSource.incidental),
-    ]);
+    await db.batch((b) {
+      b.insertAll(db.audioCacheIndex, [
+        AudioCacheIndexCompanion.insert(
+            trackId: 'a',
+            path: 'p',
+            sizeBytes: 100,
+            source: CacheSource.manual),
+        AudioCacheIndexCompanion.insert(
+            trackId: 'b',
+            path: 'p',
+            sizeBytes: 250,
+            source: CacheSource.incidental),
+      ]);
+    });
     expect(await mgr.usageBytes(), 350);
   });
 
@@ -55,18 +63,20 @@ void main() {
     final f2 = File('${tmp.path}/audio_cache/man.mp3');
     await f2.create(recursive: true);
     await f2.writeAsBytes(List.filled(100, 0));
-    await db.into(db.audioCacheIndex).insertAll([
-      AudioCacheIndexCompanion.insert(
-          trackId: 'inc',
-          path: f1.path,
-          sizeBytes: 100,
-          source: CacheSource.incidental),
-      AudioCacheIndexCompanion.insert(
-          trackId: 'man',
-          path: f2.path,
-          sizeBytes: 100,
-          source: CacheSource.manual),
-    ]);
+    await db.batch((b) {
+      b.insertAll(db.audioCacheIndex, [
+        AudioCacheIndexCompanion.insert(
+            trackId: 'inc',
+            path: f1.path,
+            sizeBytes: 100,
+            source: CacheSource.incidental),
+        AudioCacheIndexCompanion.insert(
+            trackId: 'man',
+            path: f2.path,
+            sizeBytes: 100,
+            source: CacheSource.manual),
+      ]);
+    });
     expect(await mgr.usageBytes(), 200);
     await mgr.evict(targetBytes: 100);
     expect(await mgr.isCached('inc'), false);