📦 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/es-iterator-helpers/Iterator.zipKeyed/shim.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var getPolyfill = require('./polyfill');
+var define = require('define-properties');
+
+var getIteratorPolyfill = require('../Iterator/polyfill');
+
+module.exports = function shimIteratorZipKeyed() {
+	var $Iterator = getIteratorPolyfill();
+	var polyfill = getPolyfill();
+	define(
+		$Iterator,
+		{ zipKeyed: polyfill },
+		{ zipKeyed: function () { return $Iterator.zipKeyed !== polyfill; } }
+	);
+
+	return polyfill;
+};