📦 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/memfs/lib/setTimeoutUnref.d.ts

@@ -0,0 +1,7 @@
+export type TSetTimeout = (callback: (...args: any[]) => void, time?: number, args?: any[]) => any;
+/**
+ * `setTimeoutUnref` is just like `setTimeout`,
+ * only in Node's environment it will "unref" its macro task.
+ */
+declare function setTimeoutUnref(callback: any, time?: any, args?: any): object;
+export default setTimeoutUnref;