fix: align schedule create/update conflict responses with documentation

The conflict responses should be send with an HTTP 409 status code as
they’re defined in our documentation. This was not the case as the
dashboard couldn’t handle these error codes, but is now fixed in
dashboard@4a07ad9e.

program/views.py

@@ -487,8 +487,7 @@ class APIScheduleViewSet(
         try:
             resolution = Schedule.resolve_conflicts(request.data, pk, show_pk)
         except ScheduleConflictError as exc:
-            # TODO: respond with status.HTTP_409_CONFLICT when the dashboard can handle it
-            return Response(exc.conflicts)
+            return Response(exc.conflicts, status.HTTP_409_CONFLICT)
 
         if all(key in resolution for key in ["create", "update", "delete"]):
             # this is a dry-run
@@ -535,8 +534,7 @@ class APIScheduleViewSet(
                 request.data, schedule.pk, schedule.show.pk
             )
         except ScheduleConflictError as exc:
-            # TODO: respond with status.HTTP_409_CONFLICT when the dashboard can handle it
-            return Response(exc.conflicts)
+            return Response(exc.conflicts, status.HTTP_409_CONFLICT)
 
         return Response(resolution)