📦 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/cssdb/README.md

@@ -0,0 +1,35 @@
+# cssdb [<img src="https://cssdb.org/images/cssdb.svg" alt="cssdb logo" width="90" height="90" align="right">][cssdb]
+
+[![NPM Version][npm-img]][npm-url]
+[![Build Status][cli-img]][cli-url]
+
+[cssdb] is a comprehensive list of CSS features and their positions in
+the process of becoming implemented web standards.
+
+---
+
+Did you come here to update the status of a CSS feature or add a new one?
+Quick, read [CONTRIBUTING.md](CONTRIBUTING.md).
+
+Did you come here to learn about the stages? Quick, read [STAGES.md](STAGES.md).
+
+---
+
+[cssdb] ranks CSS features by stages that reflect the real-life stability of
+new CSS features.
+
+You can read an [inside view of the CSSWG] to learn about the official
+(and unofficial) development stages of CSS specifications. In reality,
+specifications and browser implementations happen out of sync. For example,
+there are stable CSS features missing in all browsers, while other CSS features
+developed outside the [CSSWG] have appeared in browsers behind flags. This is
+too ambiguous for the web development community, and a more accountable process
+is desired.
+
+[cli-img]: https://github.com/csstools/cssdb/actions/workflows/test.yml/badge.svg
+[cli-url]: https://github.com/csstools/cssdb/actions/workflows/test.yml
+[cssdb]: https://github.com/csstools/cssdb
+[CSSWG]: https://wiki.csswg.org/spec
+[inside view of the CSSWG]: https://fantasai.inkedblade.net/weblog/2011/inside-csswg/process
+[npm-img]: https://img.shields.io/npm/v/cssdb.svg
+[npm-url]: https://www.npmjs.com/package/cssdb