fix: venv改为懒加载，兼容嵌入式CPython无venv模块

services/pyenv_manager.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 """SenSu PyEnvManager — Python 版本管理 + venv + 依赖安装"""
-import os, sys, subprocess, logging, venv, shutil
+import os, sys, subprocess, logging, shutil
 from pathlib import Path
 from typing import Optional, List
 
@@ -76,7 +76,8 @@ class PyEnvManager:
         logger.info(f"创建 venv: {venv_path} (Python {python_version or 'default'})")
 
         try:
-            venv.create(str(venv_path), with_pip=True, clear=True)
+            import venv as _venv
+            _venv.create(str(venv_path), with_pip=True, clear=True)
             # Install/upgrade pip
             pip = str(venv_path / "bin" / "pip")
             subprocess.run([pip, "install", "--upgrade", "pip"], capture_output=True, timeout=60)