📦 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.array.group-by-to-map.js

@@ -0,0 +1,16 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var $ = require('../internals/export');
+var arrayMethodIsStrict = require('../internals/array-method-is-strict');
+var addToUnscopables = require('../internals/add-to-unscopables');
+var $groupToMap = require('../internals/array-group-to-map');
+var IS_PURE = require('../internals/is-pure');
+
+// `Array.prototype.groupByToMap` method
+// https://github.com/tc39/proposal-array-grouping
+// https://bugs.webkit.org/show_bug.cgi?id=236541
+$({ target: 'Array', proto: true, name: 'groupToMap', forced: IS_PURE || !arrayMethodIsStrict('groupByToMap') }, {
+  groupByToMap: $groupToMap
+});
+
+addToUnscopables('groupByToMap');