feat(database): create central list for all database migrations

- Introduce `allMigrations` list to store all database migration classes
- Add initial migration for refactoring ad config to role-based system
- Include necessary imports for migration functionality

Author: fulleni

lib/src/database/migrations/all_migrations.dart

@@ -0,0 +1,11 @@
+import 'package:flutter_news_app_api_server_full_source_code/src/database/migration.dart';
+import 'package:flutter_news_app_api_server_full_source_code/src/database/migrations/20250924084800__refactor_ad_config_to_role_based.dart';
+
+/// A central list of all database migrations to be applied.
+///
+/// New migration classes should be added to this list in the order they are
+/// created. The [DatabaseMigrationService] will automatically sort and apply
+/// them based on their version.
+final List<Migration> allMigrations = [
+  RefactorAdConfigToRoleBased(),
+];