Refactor statistics repository to use created_at for historical data retrieval

- Updated the `Statistics` method to utilize `created_at` instead of `processing_metadata.scraped_at` for fetching daily counts, ensuring accurate historical data representation.
- Removed redundant conditions in the `scrapedDocumentsPerDay` method, streamlining the query logic for better performance and clarity.
- Added a new index on `source` and `created_at` to optimize database queries related to scraped documents.

This update enhances the accuracy of data retrieval in the dashboard statistics, improving the overall efficiency of the tender management system.

internal/notice/repository.go

@@ -43,6 +43,10 @@ func NewRepository(mongoManager *orm.ConnectionManager, logger logger.Logger) Re
 			{Key: "source", Value: 1},
 			{Key: "processing_metadata.scraped_at", Value: 1},
 		}),
+		*orm.NewIndex("source_created_at_idx", bson.D{
+			{Key: "source", Value: 1},
+			{Key: "created_at", Value: 1},
+		}),
 		*orm.NewIndex("main_classification_idx", bson.D{{Key: "main_classification", Value: 1}}),
 		*orm.NewIndex("contract_notice_id_idx", bson.D{{Key: "contract_notice_id", Value: 1}}),
 		// One row per TED ContractNoticeID when present (parallel scrape races).