📦 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-background-sync/QueueStore.js

@@ -0,0 +1,12 @@
+/*
+  Copyright 2021 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 './_version.js';
+// This is a temporary workaround to expose something from ./lib/ via our
+// top-level public API.
+// TODO: In Workbox v7, move the actual code from ./lib/ to this file.
+export { QueueStore } from './lib/QueueStore';