📦 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-webpack-plugin/src/index.ts

@@ -0,0 +1,19 @@
+/*
+  Copyright 2018 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 {GenerateSW, GenerateSWConfig} from './generate-sw';
+import {InjectManifest} from './inject-manifest';
+
+/**
+ * @module workbox-webpack-plugin
+ */
+export {GenerateSW, GenerateSWConfig, InjectManifest};
+
+// TODO: remove this in v7.
+// See https://github.com/GoogleChrome/workbox/issues/3033
+export default {GenerateSW, InjectManifest};