simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/queries/timetable_custom_events/timetable_custom_events_get.dart

@@ -4,10 +4,8 @@ import '../../marianumconnect_query.dart';
 class TimetableCustomEventsGet extends MarianumConnectQuery {
   TimetableCustomEventsGet({super.dio});
 
-  Future<GetCustomTimetableEventResponse> run() => guard(() async {
-    final response = await dio.get<Map<String, dynamic>>(
-      endpoint('timetable/custom-events'),
-    );
-    return GetCustomTimetableEventResponse.fromJson(response.data!);
-  });
+  Future<GetCustomTimetableEventResponse> run() => getObject(
+    'timetable/custom-events',
+    GetCustomTimetableEventResponse.fromJson,
+  );
 }