🔒️(backend) validate Room configuration with Pydantic schema

Room.configuration accepted arbitrary JSON without validation, allowing unsafe
or malformed payloads to be stored and creating a security risk. Define a
Pydantic schema to enforce structure and constraints, and add validation
at the serializer level to reject invalid inputs.

src/backend/core/services/lobby.py

@@ -123,7 +123,7 @@ class LobbyService:
 
     def request_entry(
         self,
-        room,
+        room: models.Room,
         request,
         username: str,
     ) -> Tuple[LobbyParticipant, Optional[Dict]]: