📦 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/babel-preset-current-node-syntax/README.md

@@ -0,0 +1,30 @@
+# `babel-preset-current-node-syntax`
+
+> A Babel preset that enables parsing of proposals supported by the current Node.js version.
+
+## Installation
+
+If you are using yarn:
+```
+yarn add --dev babel-preset-current-node-syntax
+```
+
+If you are using npm:
+```
+npm install --save-dev babel-preset-current-node-syntax
+```
+
+## Contributing
+
+PRs are welcome! The codebase is so small that I didn't setup a linter, but try
+to match the style of the existing code.
+
+You can run tests with the following command:
+```
+yarn node test/index.js
+```
+
+The `test/fixtures.json` file contains a bunch of syntax tests, alongside with
+the minimum supported node version for each of them. Babel should throw on
+older versions, without support for that given syntax.
+All the tests are run using `@babel/parser@7.0.0`.