refactor(api): use repository count method for dashboard summary

Replaces the `readAll()` calls in the `DashboardSummaryService` with the more efficient `count()` method from the data repository.

This avoids fetching all documents from the database just to get their count, significantly improving the performance of the dashboard summary endpoint by leveraging the native counting capabilities of the database.

Author: fulleni

lib/src/services/dashboard_summary_service.dart

@@ -23,26 +23,26 @@ class DashboardSummaryService {
 
   /// Calculates and returns the current dashboard summary.
   ///
-  /// This method fetches all items from the required repositories to count them
-  /// and constructs a [DashboardSummary] object.
+  /// This method fetches the counts of all items from the required
+  /// repositories and constructs a [DashboardSummary] object.
   Future<DashboardSummary> getSummary() async {
     // Use Future.wait to fetch all counts in parallel for efficiency.
     final results = await Future.wait([
-      _headlineRepository.readAll(),
-      _topicRepository.readAll(),
-      _sourceRepository.readAll(),
+      _headlineRepository.count(),
+      _topicRepository.count(),
+      _sourceRepository.count(),
     ]);
 
-    // The results are PaginatedResponse objects.
-    final headlineResponse = results[0] as PaginatedResponse<Headline>;
-    final topicResponse = results[1] as PaginatedResponse<Topic>;
-    final sourceResponse = results[2] as PaginatedResponse<Source>;
+    // The results are integers.
+    final headlineCount = results[0];
+    final topicCount = results[1];
+    final sourceCount = results[2];
 
     return DashboardSummary(
       id: 'dashboard_summary', // Fixed ID for the singleton summary
-      headlineCount: headlineResponse.items.length,
-      topicCount: topicResponse.items.length,
-      sourceCount: sourceResponse.items.length,
+      headlineCount: headlineCount,
+      topicCount: topicCount,
+      sourceCount: sourceCount,
     );
   }
 }