📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/workbox-precaching/utils/getOrCreatePrecacheController.js

@@ -0,0 +1,20 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { PrecacheController } from '../PrecacheController.js';
+import '../_version.js';
+let precacheController;
+/**
+ * @return {PrecacheController}
+ * @private
+ */
+export const getOrCreatePrecacheController = () => {
+    if (!precacheController) {
+        precacheController = new PrecacheController();
+    }
+    return precacheController;
+};