api: reduce search window for open slots in fillslot (#56896)

chrono/api/views.py

@@ -1283,6 +1283,8 @@ class Fillslots(APIView):
                     meeting_type,
                     resources=resources,
                     user_external_id=user_external_id if exclude_user else None,
+                    start_datetime=min(datetimes),
+                    end_datetime=max(datetimes) + datetime.timedelta(minutes=meeting_type.duration),
                 ),
                 key=lambda slot: slot.start_datetime,
             )