feat(database): add saved filters limit to remote config

- Introduce new fields for saved filters limits in remote config
- Set default values for guest, authenticated, and premium user tiers
- Implement migration to add these fields to existing documents
- Create reversible migration with up and down methods

Author: fulleni

lib/src/database/migrations/20251013000057_add_saved_filters_to_remote_config.dart

@@ -0,0 +1,52 @@
+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 add the `savedFiltersLimit` fields to existing
+/// `remote_configs` documents within the `userPreferenceConfig` sub-document.
+class AddSavedFiltersToRemoteConfig extends Migration {
+  /// {@macro add_saved_filters_to_remote_config}
+  AddSavedFiltersToRemoteConfig()
+    : super(
+        prDate: '20251013000057',
+        prId: '57',
+        prSummary:
+            'This pull request introduces the ability for users to save and manage custom filter combinations for news headlines. It achieves this by adding a new SavedFilter data model, integrating it into the existing user content preferences, and implementing configurable limits for these saved filters based on user tiers',
+      );
+
+  @override
+  Future<void> up(Db db, Logger log) async {
+    final collection = db.collection('remote_configs');
+    final result = await collection.updateMany(
+      // Filter for documents where 'userPreferenceConfig.guestSavedFiltersLimit' does not exist.
+      // This assumes if one is missing, all are likely missing.
+      where.notExists('userPreferenceConfig.guestSavedFiltersLimit'),
+      // Set 'guestSavedFiltersLimit', 'authenticatedSavedFiltersLimit',
+      // and 'premiumSavedFiltersLimit' to a default value.
+      modify
+          .set('userPreferenceConfig.guestSavedFiltersLimit', 3)
+          .set('userPreferenceConfig.authenticatedSavedFiltersLimit', 10)
+          .set('userPreferenceConfig.premiumSavedFiltersLimit', 25),
+    );
+    log.info(
+      'Updated ${result.nModified} documents in remote_configs '
+      'to include savedFiltersLimit fields.',
+    );
+  }
+
+  @override
+  Future<void> down(Db db, Logger log) async {
+    final collection = db.collection('remote_configs');
+    await collection.updateMany(
+      where.exists('userPreferenceConfig.guestSavedFiltersLimit'),
+      modify
+          .unset('userPreferenceConfig.guestSavedFiltersLimit')
+          .unset('userPreferenceConfig.authenticatedSavedFiltersLimit')
+          .unset('userPreferenceConfig.premiumSavedFiltersLimit'),
+    );
+    log.info(
+      'Removed "savedFiltersLimit" fields from remote_configs '
+      'userPreferenceConfig sub-document.',
+    );
+  }
+}