feat(api): create dashboard summary service structure

Author: fulleni

lib/src/services/dashboard_summary_service.dart

@@ -0,0 +1,41 @@
+import 'package:ht_data_repository/ht_data_repository.dart';
+import 'package:ht_shared/ht_shared.dart';
+
+/// {@template dashboard_summary_service}
+/// A service responsible for calculating the dashboard summary data on demand.
+///
+/// This service aggregates data from various repositories to provide a
+/// real-time overview of key metrics in the system.
+/// {@endtemplate}
+class DashboardSummaryService {
+  /// {@macro dashboard_summary_service}
+  const DashboardSummaryService({
+    required HtDataRepository<User> userRepository,
+    required HtDataRepository<Headline> headlineRepository,
+    required HtDataRepository<Category> categoryRepository,
+    required HtDataRepository<Source> sourceRepository,
+  })  : _userRepository = userRepository,
+        _headlineRepository = headlineRepository,
+        _categoryRepository = categoryRepository,
+        _sourceRepository = sourceRepository;
+
+  final HtDataRepository<User> _userRepository;
+  final HtDataRepository<Headline> _headlineRepository;
+  final HtDataRepository<Category> _categoryRepository;
+  final HtDataRepository<Source> _sourceRepository;
+
+  /// Calculates and returns the current dashboard summary.
+  ///
+  /// This method fetches all items from the required repositories to count them
+  /// and constructs a [DashboardSummary] object.
+  Future<DashboardSummary> getSummary() async {
+    // The actual calculation logic will be implemented in a subsequent step.
+    // For now, this serves as a placeholder structure.
+    return DashboardSummary(
+      id: 'dashboard_summary',
+      headlineCount: 0,
+      categoryCount: 0,
+      sourceCount: 0,
+    );
+  }
+}