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/fastapi/middleware/asyncexitstack.py

@@ -0,0 +1,18 @@
+from contextlib import AsyncExitStack
+
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+
+# Used mainly to close files after the request is done, dependencies are closed
+# in their own AsyncExitStack
+class AsyncExitStackMiddleware:
+    def __init__(
+        self, app: ASGIApp, context_name: str = "fastapi_middleware_astack"
+    ) -> None:
+        self.app = app
+        self.context_name = context_name
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        async with AsyncExitStack() as stack:
+            scope[self.context_name] = stack
+            await self.app(scope, receive, send)