📦 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/binary-extensions/readme.md

@@ -0,0 +1,25 @@
+# binary-extensions
+
+> List of binary file extensions
+
+The list is just a [JSON file](binary-extensions.json) and can be used anywhere.
+
+## Install
+
+```sh
+npm install binary-extensions
+```
+
+## Usage
+
+```js
+const binaryExtensions = require('binary-extensions');
+
+console.log(binaryExtensions);
+//=> ['3ds', '3g2', …]
+```
+
+## Related
+
+- [is-binary-path](https://github.com/sindresorhus/is-binary-path) - Check if a filepath is a binary file
+- [text-extensions](https://github.com/sindresorhus/text-extensions) - List of text file extensions