📦 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/postcss-js/README.md

@@ -0,0 +1,22 @@
+# PostCSS JS
+
+<img align="right" width="135" height="95"
+     title="Philosopher’s stone, logo of PostCSS"
+     src="https://postcss.org/logo-leftp.svg">
+
+[PostCSS] for CSS-in-JS and styles in JS objects.
+
+For example, to use [Stylelint] or [RTLCSS] plugins in your workflow.
+
+<a href="https://evilmartians.com/?utm_source=postcss-js">
+  <img src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg"
+       alt="Sponsored by Evil Martians" width="236" height="54">
+</a>
+
+[Stylelint]: https://github.com/stylelint/stylelint
+[PostCSS]:   https://github.com/postcss/postcss
+[RTLCSS]:    https://github.com/MohammadYounes/rtlcss
+
+
+## Docs
+Read full docs **[here](https://github.com/postcss/postcss-js#readme)**.