📦 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-minify-selectors/README.md

@@ -0,0 +1,42 @@
+# [postcss][postcss]-minify-selectors
+
+> Minify selectors with PostCSS.
+
+## Install
+
+With [npm](https://www.npmjs.com/package/postcss-minify-selectors) do:
+
+```
+npm install postcss-minify-selectors --save
+```
+
+## Example
+
+### Input
+
+```css
+h1 + p, h2, h3, h2{color:blue}
+```
+
+### Output
+
+```css
+h1+p,h2,h3{color:blue}
+```
+
+For more examples see the [tests](test.js).
+
+## Usage
+
+See the [PostCSS documentation](https://github.com/postcss/postcss#usage) for
+examples for your environment.
+
+## Contributors
+
+See [CONTRIBUTORS.md](https://github.com/cssnano/cssnano/blob/master/CONTRIBUTORS.md).
+
+## License
+
+MIT © [Ben Briggs](http://beneb.info)
+
+[postcss]: https://github.com/postcss/postcss