Enhance notification search functionality with user ID filtering

- Added a new `UserID` field to the `SearchForm` to allow filtering notifications by user ID.
- Implemented the `ResolvedRecipients` method to return user IDs based on the `user_id` or `recipient` query parameters, improving the flexibility of notification searches.
- Updated the `GetNotifications` method in the service layer to utilize the new recipient resolution logic, ensuring accurate retrieval of notifications based on user ID.

This update enhances the notification management capabilities, providing more granular control over notification searches.

internal/notification/handler.go

@@ -65,6 +65,7 @@ func (h *NotificationHandler) Send(c echo.Context) error {
 // @Param event_type query string false "Event type filter"
 // @Param type query string false "Type filter"
 // @Param recipient query string false "Recipient filter"
+// @Param user_id query string false "User ID filter"
 // @Param seen query bool false "Seen filter"
 // @Param priority query string false "Priority filter"
 // @Param limit query int false "Limit results"