Implement deadline matching logic for dashboard repository and enhance unit tests

- Added the `deadlineWithinWindowMatch` function to match raw deadline fields stored as Unix seconds or milliseconds, ensuring accurate filtering in MongoDB queries.
- Updated the `ClosingSoon` method in the repository to utilize the new deadline matching logic, improving query accuracy for upcoming deadlines.
- Introduced a new unit test, `TestDeadlineWithinWindowMatchCoversSecondsAndMilliseconds`, to validate the functionality of the deadline matching logic, ensuring both seconds and milliseconds are correctly handled.
- Enhanced comments in the `widget_cache` and `search_list_cache` files for better clarity on cache behavior and staleness.

This update improves the accuracy of deadline handling in the dashboard service and strengthens the test coverage for related functionalities.

internal/tender/search_list_cache.go

@@ -12,7 +12,9 @@ import (
 )
 
 const (
-	searchListCacheTTL       = 60 * time.Second
+	searchListCacheTTL = 60 * time.Second
+	// searchListCacheStaleTTL bounds how long the default unfiltered list may lag after
+	// writes. Only isCacheableSearchList queries use this cache (~5 min max staleness).
 	searchListCacheStaleTTL  = 5 * time.Minute
 	searchListCacheKeyPrefix = "tender:search:list:"
 )
@@ -92,6 +94,8 @@ func (s *tenderService) cachedSearchList(ctx context.Context, form *SearchForm,
 		return load(ctx)
 	}
 
+	// Cached *SearchResponse values (including the Tenders slice) are shared across
+	// concurrent callers via singleflight and stale hits; treat as read-only.
 	cacheKey := searchListCacheKey(pagination, language)
 	redisKey := searchListCacheKeyPrefix + cacheKey