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/dashboard/widget_cache.go

@@ -14,7 +14,11 @@ import (
 )
 
 const (
-	widgetCacheTTL      = 60 * time.Second
+	// widgetCacheTTL is the fresh window before a background reload is triggered.
+	widgetCacheTTL = 60 * time.Second
+	// widgetStaleGraceTTL is how long stale entries may be served while reloading.
+	// Dashboard widgets have no write-side invalidation; expect up to this lag after
+	// data changes (e.g. a new tender may not appear in cached widgets immediately).
 	widgetStaleGraceTTL = 5 * time.Minute
 	widgetReloadTimeout = 2 * time.Minute
 )
@@ -46,6 +50,8 @@ func newWidgetCache[T any](redisClient redis.Client, log logger.Logger, keyPrefi
 func (c *widgetCache[T]) Get(ctx context.Context, key string, load widgetLoader[T]) (T, error) {
 	var zero T
 
+	// Returned values are shared across concurrent callers via singleflight and stale
+	// hits. Callers must treat them as read-only through JSON serialization.
 	if value, ok := c.fresh(key); ok {
 		return value, nil
 	}