fix: 兼容 Python 3.8 类型注解语法 (from __future__ import annotations)

backend/app/hosts.py

@@ -1,9 +1,10 @@
+from __future__ import annotations
 """主机注册表 - 管理多台 libvirt 宿主机"""
 import json
 import os
 import time
-import uuid
 import logging
+import uuid
 from typing import Optional
 from pydantic import BaseModel, Field
 from app.config import settings

backend/app/libvirt_conn.py

@@ -1,4 +1,5 @@
 """libvirt 多主机连接管理器"""
+from __future__ import annotations
 import libvirt
 from contextlib import contextmanager
 import logging