fix: apply replay gain preferences on scheduled files (#2945)

### Description

The replay gain preferences are applied in the legacy code, but the
playout code was missing this feature. The replay gain was not applied
when playout fetched the schedules.


37d1a7685e/legacy/application/models/Schedule.php (L881-L886)

api/libretime_api/core/views/stream.py

@@ -19,6 +19,8 @@ class StreamPreferencesView(views.APIView):
                     "input_fade_transition",
                     "message_format",
                     "message_offline",
+                    "replay_gain_enabled",
+                    "replay_gain_offset",
                 }
             )
         )