fix(qa): api.py 补回 import os(backend 段依赖,移植时误丢)

src/rag/qa_v1_1_73/api.py

@@ -5,6 +5,7 @@ import json
 import queue
 import threading
 from functools import lru_cache
+import os
 from typing import Any, Dict, Optional
 
 from fastapi import APIRouter, HTTPException