📦 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/performance-now/test/scripts/initial-value.coffee

@@ -0,0 +1,10 @@
+#!/usr/bin/env ./node_modules/.bin/coffee
+
+###
+Expected output is a number above 100 and below 350.
+The time reported is relative to the time the node.js process was started
+this is approximately at `(Date.now() process.uptime() * 1000)`
+###
+
+now = require '../../lib/performance-now'
+console.log now().toFixed 3