📦 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/modules/esnext.symbol.is-well-known.js

@@ -0,0 +1,10 @@
+'use strict';
+var $ = require('../internals/export');
+var isWellKnownSymbol = require('../internals/symbol-is-well-known');
+
+// `Symbol.isWellKnown` method
+// obsolete version of https://tc39.es/proposal-symbol-predicates/#sec-symbol-iswellknownsymbol
+// We should patch it for newly added well-known symbols. If it's not required, this module just will not be injected
+$({ target: 'Symbol', stat: true, name: 'isWellKnownSymbol', forced: true }, {
+  isWellKnown: isWellKnownSymbol
+});