fix(mpc-system): 开发模式添加 message-router gRPC 端口映射

添加 50051:50051 端口映射，使开发模式与生产模式保持一致

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/mpc-system/docker-compose.yml

@@ -91,7 +91,8 @@ services:
       dockerfile: services/message-router/Dockerfile
     container_name: mpc-message-router
     ports:
-      - "8082:8080"  # WebSocket for external connections
+      - "50051:50051"  # gRPC for party connections
+      - "8082:8080"    # HTTP for health checks
     environment:
       TZ: Asia/Shanghai
       MPC_SERVER_GRPC_PORT: 50051