Enhance dashboard repository and service with caching and aggregation improvements

- Introduced caching mechanisms for summary and statistics in the dashboard service, improving performance by reducing redundant data retrieval.
- Refactored the Summary method to utilize MongoDB aggregation for more efficient data processing and retrieval.
- Added synchronization features using singleflight to prevent duplicate processing of requests for cached data.
- Updated the repository to include a cachedScrapedTendersScope method, enhancing the efficiency of scraped document statistics retrieval.

This update significantly optimizes the dashboard's performance and data handling capabilities, ensuring faster response times and reduced load on the database.

internal/dashboard/statistics_repository_test.go

@@ -129,3 +129,13 @@ func TestResolveScrapedTendersScopeMongoFallbackWithoutLister(t *testing.T) {
 		t.Fatalf("expected mongo fallback filter, got %#v", scope.match)
 	}
 }
+
+func TestMongoScrapedTendersScopeMatchesStatisticsFilter(t *testing.T) {
+	scope := mongoScrapedTendersScope()
+	if scope.zero {
+		t.Fatal("expected non-zero mongo statistics scope")
+	}
+	if scope.match["processing_metadata.documents_scraped"] != true {
+		t.Fatalf("expected documents_scraped filter, got %#v", scope.match)
+	}
+}