fix: upgrade workmanager 0.5.2 → 0.9.0+3 to fix Kotlin 2.x compile error

workmanager 0.5.2 references the removed PluginRegistrantCallback and
other V1 Flutter embedding APIs, causing a Kotlin compilation failure
when building the release AAB with AGP 8.11.1 / Kotlin 2.2.20.

0.9.x uses only V2 embedding. API migration: ExistingWorkPolicy →
ExistingPeriodicWorkPolicy for registerPeriodicTask.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/sync/background_sync.dart

@@ -38,7 +38,7 @@ Future<void> registerBackgroundSync() async {
     _kTaskName,
     frequency: const Duration(minutes: 15),
     constraints: Constraints(networkType: NetworkType.connected),
-    existingWorkPolicy: ExistingWorkPolicy.keep,
+    existingWorkPolicy: ExistingPeriodicWorkPolicy.keep,
   );
 }
 

pubspec.yaml

@@ -47,7 +47,7 @@ dependencies:
 
   # Background sync and local notifications
   flutter_local_notifications: ^18.0.1
-  workmanager: ^0.5.2
+  workmanager: ^0.9.0
 
 dev_dependencies:
   flutter_test: