fix: condición de carrera al unirse múltiples jugadores

- Agrega sistema de locks en Redis para operaciones de sala
- add_player, remove_player y change_team ahora son atómicos
- Previene sobrescritura de estado cuando jugadores se unen simultáneamente
- Nuevo método change_player_team con lock integrado

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/sockets/game_events.py

@@ -189,14 +189,13 @@ def register_socket_events(sio: socketio.AsyncServer):
 
         room_code = player["room"]
         new_team = data.get("team")
-        current_team = player["team"]
 
-        # Don't do anything if already on that team
-        if current_team == new_team:
-            return
+        # Use room_manager method with lock to prevent race conditions
+        room = await room_manager.change_player_team(
+            room_code, player["name"], sid, new_team
+        )
 
-        room = await room_manager.get_room(room_code)
-        if not room or len(room["teams"][new_team]) >= 4:
+        if not room:
             await sio.emit(
                 "error",
                 {"message": "Cannot change team. It may be full."},
@@ -204,30 +203,6 @@ def register_socket_events(sio: socketio.AsyncServer):
             )
             return
 
-        # Remove from current team (by socket_id to be safe)
-        room["teams"][current_team] = [
-            p for p in room["teams"][current_team] if p["socket_id"] != sid
-        ]
-
-        # Also remove from new team if somehow already there (prevent duplicates)
-        room["teams"][new_team] = [
-            p for p in room["teams"][new_team] if p["socket_id"] != sid
-        ]
-
-        # Add to new team
-        room["teams"][new_team].append({
-            "name": player["name"],
-            "team": new_team,
-            "position": len(room["teams"][new_team]),
-            "socket_id": sid
-        })
-
-        # Update room state
-        await room_manager.update_room(room_code, room)
-
-        # Update player record with new team
-        await room_manager.update_player(sid, {"team": new_team})
-
         await sio.emit("team_changed", {"room": room}, room=room_code)
 
     @sio.event