chore: 添加虚拟环境到仓库

- 添加 backend_service/venv 虚拟环境
- 包含所有Python依赖包
- 注意：虚拟环境约393MB，包含12655个文件

backend_service/venv/lib/python3.13/site-packages/websocket/tests/echo-server.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+
+# From https://github.com/aaugustin/websockets/blob/main/example/echo.py
+
+import asyncio
+import os
+
+import websockets
+
+LOCAL_WS_SERVER_PORT = int(os.environ.get("LOCAL_WS_SERVER_PORT", "8765"))
+
+
+async def echo(websocket):
+    async for message in websocket:
+        await websocket.send(message)
+
+
+async def main():
+    async with websockets.serve(echo, "localhost", LOCAL_WS_SERVER_PORT):
+        await asyncio.Future()  # run forever
+
+
+asyncio.run(main())