🐛(backend) fix lobby notification type error breaking participant alerts

Correct data type issue that was preventing lobby notifications from
being sent to other participants in the room.

src/backend/core/services/lobby.py

@@ -212,7 +212,7 @@ class LobbyService:
 
         try:
             utils.notify_participants(
-                room_name=room_id,
+                room_name=str(room_id),
                 notification_data={
                     "type": settings.LOBBY_NOTIFICATION_TYPE,
                 },

src/backend/core/tests/services/test_lobby.py

@@ -442,7 +442,7 @@ def test_enter_success(
         timeout=settings.LOBBY_WAITING_TIMEOUT,
     )
     mock_notify.assert_called_once_with(
-        room_name=room.id, notification_data={"type": "participantWaiting"}
+        room_name=str(room.id), notification_data={"type": "participantWaiting"}
     )