Return iterator from query for speed

app/dao/notifications_dao.py

@@ -225,7 +225,7 @@ def get_notification_billable_unit_count_per_month(service_id, year):
         NotificationHistory.service_id == service_id,
         NotificationHistory.created_at >= start,
         NotificationHistory.created_at < end
-    ).all()
+    )
 
     return [
         (month, sum(count for _, count in row))