fix: lifespan 关闭时 await 取消的采集任务，避免 pending 警告

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

backend/main.py

@@ -21,6 +21,10 @@ async def lifespan(app: FastAPI):
         yield
     finally:
         collector_task.cancel()
+        try:
+            await collector_task
+        except asyncio.CancelledError:
+            pass
 
 
 app = FastAPI(title="AUV API", version="1.0.0", lifespan=lifespan)