📦 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/core-js-pure/internals/a-map.js

@@ -0,0 +1,10 @@
+'use strict';
+var tryToString = require('../internals/try-to-string');
+
+var $TypeError = TypeError;
+
+// Perform ? RequireInternalSlot(M, [[MapData]])
+module.exports = function (it) {
+  if (typeof it == 'object' && 'size' in it && 'has' in it && 'get' in it && 'set' in it && 'delete' in it && 'entries' in it) return it;
+  throw new $TypeError(tryToString(it) + ' is not a map');
+};