refactor(db): adapt user seeding to new preference models

Updates the `_createUserSubDocuments` method in `DatabaseSeedingService` to use the new `UserContentPreferences` constructor. This removes the deprecated `interests` field and adds the required `savedHeadlineFilters` and `savedSourceFilters` fields, ensuring default user data is correctly structured.

Author: fulleni

lib/src/services/database_seeding_service.dart

@@ -389,13 +389,15 @@ class DatabaseSeedingService {
       ...defaultAppSettings.toJson()..remove('id'),
     });
 
+    // Initialize with empty lists for all user-managed content.
     final defaultUserPreferences = UserContentPreferences(
       id: userId.oid,
       followedCountries: const [],
       followedSources: const [],
       followedTopics: const [],
       savedHeadlines: const [],
-      interests: const [],
+      savedHeadlineFilters: const [],
+      savedSourceFilters: const [],
     );
     await _db.collection('user_content_preferences').insertOne({
       '_id': userId,