feat(music): android background playback via media_controls foreground service

Bumps os_media_controls to 4f4b28f3: MediaStyle foreground service with
a JUnit-tested promote/demote/stop policy, artwork URL download (also
fixes video lock-screen art), and task-removal teardown that can't leak
orphan notifications. The music service opts into background mode per
session and requests POST_NOTIFICATIONS before first playback.

lib/utils/notification_permission.dart

@@ -0,0 +1,31 @@
+import 'dart:io';
+
+import 'package:background_downloader/background_downloader.dart';
+
+import 'app_logger.dart';
+
+bool _requested = false;
+
+/// Best-effort request for the Android 13+ POST_NOTIFICATIONS runtime
+/// permission (routed through background_downloader's permissions API, which
+/// the app already ships for download notifications).
+///
+/// Needed so the background music playback notification is visible; playback
+/// and its foreground service run regardless of the outcome, so a denial only
+/// costs notification visibility. Asked at most once per app run — Android
+/// remembers a real denial, so re-prompting is a no-op anyway.
+///
+/// No-op off Android: iOS/macOS media controls don't use notifications.
+Future<void> ensureNotificationPermission() async {
+  if (_requested || !Platform.isAndroid) return;
+  _requested = true;
+  try {
+    final permissions = FileDownloader().permissions;
+    final status = await permissions.status(PermissionType.notifications);
+    if (status == PermissionStatus.granted) return;
+    final result = await permissions.request(PermissionType.notifications);
+    appLogger.d('Notification permission request result: $result');
+  } catch (e) {
+    appLogger.w('Notification permission request failed', error: e);
+  }
+}