refactor(database): remove legacy local ad platform from remote configs

- Add migration to delete 'local' ad platform from remote configs collection
- Remove 'local' key from adConfig.platformAdIdentifiers map
- Update primaryAdPlatform from 'local' to 'admob' where applicable
- Prevent deserialization errors after removing AdPlatformType.local

Author: fulleni

lib/src/database/migrations/20251103073226_remove_local_ad_platform.dart

@@ -0,0 +1,65 @@
+import 'package:flutter_news_app_api_server_full_source_code/src/database/migration.dart';
+import 'package:logging/logging.dart';
+import 'package:mongo_dart/mongo_dart.dart';
+
+/// Migration to remove the legacy `local` ad platform from the `remote_configs`
+/// collection.
+///
+/// This migration performs two critical cleanup tasks:
+/// 1. It removes the `local` key from the `adConfig.platformAdIdentifiers` map
+///    in all `remote_configs` documents.
+/// 2. It updates any `remote_configs` document where the `primaryAdPlatform`
+///    is set to `local`, changing it to `admob`.
+///
+/// This ensures data consistency after the removal of the `AdPlatformType.local`
+/// enum value and prevents deserialization errors in the application.
+class RemoveLocalAdPlatform extends Migration {
+  /// {@macro remove_local_ad_platform}
+  RemoveLocalAdPlatform()
+    : super(
+        prDate: '20251103073226',
+        prId: '57',
+        prSummary:
+            'Removes the legacy local ad platform from the remote config, migrating existing data to use admob as the default.',
+      );
+
+  @override
+  Future<void> up(Db db, Logger log) async {
+    final collection = db.collection('remote_configs');
+
+    // Step 1: Unset the 'local' key from the platformAdIdentifiers map.
+    // This removes the field entirely from any document where it exists.
+    log.info(
+      'Attempting to remove "adConfig.platformAdIdentifiers.local" field...',
+    );
+    final unsetResult = await collection.updateMany(
+      where.exists('adConfig.platformAdIdentifiers.local'),
+      modify.unset('adConfig.platformAdIdentifiers.local'),
+    );
+    log.info(
+      'Removed "adConfig.platformAdIdentifiers.local" from ${unsetResult.nModified} documents.',
+    );
+
+    // Step 2: Update the primaryAdPlatform from 'local' to 'admob'.
+    // This ensures that no document is left with an invalid primary platform.
+    log.info(
+      'Attempting to migrate primaryAdPlatform from "local" to "admob"...',
+    );
+    final updateResult = await collection.updateMany(
+      where.eq('adConfig.primaryAdPlatform', 'local'),
+      modify.set('adConfig.primaryAdPlatform', 'admob'),
+    );
+    log.info(
+      'Migrated primaryAdPlatform to "admob" for ${updateResult.nModified} documents.',
+    );
+  }
+
+  @override
+  Future<void> down(Db db, Logger log) async {
+    // Reverting this change is not safe as it would require re-introducing
+    // an enum value that no longer exists in the code.
+    log.warning(
+      'Reverting "RemoveLocalAdPlatform" is not supported. The "local" ad platform configuration would need to be manually restored if required.',
+    );
+  }
+}

lib/src/database/migrations/all_migrations.dart

@@ -3,6 +3,7 @@ import 'package:flutter_news_app_api_server_full_source_code/src/database/migrat
 import 'package:flutter_news_app_api_server_full_source_code/src/database/migrations/20251013000056_add_saved_filters_to_user_preferences.dart';
 import 'package:flutter_news_app_api_server_full_source_code/src/database/migrations/20251013000057_add_saved_filters_to_remote_config.dart';
 import 'package:flutter_news_app_api_server_full_source_code/src/database/migrations/20251024000000_add_logo_url_to_sources.dart';
+import 'package:flutter_news_app_api_server_full_source_code/src/database/migrations/20251103073226_remove_local_ad_platform.dart';
 import 'package:flutter_news_app_api_server_full_source_code/src/services/database_migration_service.dart'
     show DatabaseMigrationService;
 
@@ -16,4 +17,5 @@ final List<Migration> allMigrations = [
   AddSavedFiltersToUserPreferences(),
   AddSavedFiltersToRemoteConfig(),
   AddLogoUrlToSources(),
+  RemoveLocalAdPlatform(),
 ];