📦 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/mkdirp/bin/usage.txt

@@ -0,0 +1,12 @@
+usage: mkdirp [DIR1,DIR2..] {OPTIONS}
+
+  Create each supplied directory including any necessary parent directories that
+  don't yet exist.
+  
+  If the directory already exists, do nothing.
+
+OPTIONS are:
+
+  -m, --mode   If a directory needs to be created, set the mode as an octal
+               permission string.
+