feat: Add DemoDataMigrationService

- Migrates user data on demo login
- Handles UserAppSettings migration
- Handles UserContentPreferences migration
- Idempotent and resilient

Author: fulleni

lib/shared/services/demo_data_migration_service.dart

@@ -0,0 +1,103 @@
+import 'package:ht_data_repository/ht_data_repository.dart';
+import 'package:ht_shared/ht_shared.dart';
+
+/// {@template demo_data_migration_service}
+/// A service responsible for migrating user data (settings and preferences)
+/// from an anonymous user ID to a new authenticated user ID in demo mode.
+///
+/// This service is specifically designed for the in-memory data clients
+/// used in the demo environment, as backend APIs typically handle this
+/// migration automatically.
+/// {@endtemplate}
+class DemoDataMigrationService {
+  /// {@macro demo_data_migration_service}
+  const DemoDataMigrationService({
+    required HtDataRepository<UserAppSettings> userAppSettingsRepository,
+    required HtDataRepository<UserContentPreferences>
+        userContentPreferencesRepository,
+  })  : _userAppSettingsRepository = userAppSettingsRepository,
+        _userContentPreferencesRepository = userContentPreferencesRepository;
+
+  final HtDataRepository<UserAppSettings> _userAppSettingsRepository;
+  final HtDataRepository<UserContentPreferences>
+      _userContentPreferencesRepository;
+
+  /// Migrates user settings and content preferences from an old anonymous
+  /// user ID to a new authenticated user ID.
+  ///
+  /// This operation is designed to be idempotent and resilient to missing
+  /// data for the old user ID.
+  Future<void> migrateAnonymousData({
+    required String oldUserId,
+    required String newUserId,
+  }) async {
+    print(
+      '[DemoDataMigrationService] Attempting to migrate data from '
+      'anonymous user ID: $oldUserId to authenticated user ID: $newUserId',
+    );
+
+    // Migrate UserAppSettings
+    try {
+      final oldSettings = await _userAppSettingsRepository.read(
+        id: oldUserId,
+        userId: oldUserId,
+      );
+      final newSettings = oldSettings.copyWith(id: newUserId);
+      await _userAppSettingsRepository.update(
+        id: newUserId,
+        item: newSettings,
+        userId: newUserId,
+      );
+      await _userAppSettingsRepository.delete(
+        id: oldUserId,
+        userId: oldUserId,
+      );
+      print(
+        '[DemoDataMigrationService] UserAppSettings migrated successfully '
+        'from $oldUserId to $newUserId.',
+      );
+    } on NotFoundException {
+      print(
+        '[DemoDataMigrationService] No UserAppSettings found for old user ID: '
+        '$oldUserId. Skipping migration for settings.',
+      );
+    } catch (e, s) {
+      print(
+        '[DemoDataMigrationService] Error migrating UserAppSettings from '
+        '$oldUserId to $newUserId: $e\n$s',
+      );
+    }
+
+    // Migrate UserContentPreferences
+    try {
+      final oldPreferences = await _userContentPreferencesRepository.read(
+        id: oldUserId,
+        userId: oldUserId,
+      );
+      final newPreferences = oldPreferences.copyWith(id: newUserId);
+      await _userContentPreferencesRepository.update(
+        id: newUserId,
+        item: newPreferences,
+        userId: newUserId,
+      );
+      await _userContentPreferencesRepository.delete(
+        id: oldUserId,
+        userId: oldUserId,
+      );
+      print(
+        '[DemoDataMigrationService] UserContentPreferences migrated '
+        'successfully from $oldUserId to $newUserId.',
+      );
+    } on NotFoundException {
+      print(
+        '[DemoDataMigrationService] No UserContentPreferences found for old '
+        'user ID: $oldUserId. Skipping migration for preferences.',
+      );
+    } catch (e, s) {
+      print(
+        '[DemoDataMigrationService] Error migrating UserContentPreferences '
+        'from $oldUserId to $newUserId: $e\n$s',
+      );
+    }
+  }
+}