simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/queries/timetable_get_element_week/timetable_get_element_week.dart

@@ -13,14 +13,9 @@ class TimetableGetElementWeek extends MarianumConnectQuery {
     required int id,
     required DateTime from,
     required DateTime until,
-  }) => guard(() async {
-    final response = await dio.get<Map<String, dynamic>>(
-      endpoint('timetable/${type.pathSegment}/$id'),
-      queryParameters: {'from': _format(from), 'until': _format(until)},
-    );
-    return TimetableGetWeekResponse.fromJson(response.data!);
-  });
-
-  String _format(DateTime d) =>
-      '${d.year.toString().padLeft(4, '0')}-${d.month.toString().padLeft(2, '0')}-${d.day.toString().padLeft(2, '0')}';
+  }) => getObject(
+    'timetable/${type.pathSegment}/$id',
+    TimetableGetWeekResponse.fromJson,
+    queryParameters: {'from': isoDate(from), 'until': isoDate(until)},
+  );
 }