📦 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.composite-key.js

@@ -0,0 +1,20 @@
+'use strict';
+var $ = require('../internals/export');
+var apply = require('../internals/function-apply');
+var getCompositeKeyNode = require('../internals/composite-key');
+var getBuiltIn = require('../internals/get-built-in');
+var create = require('../internals/object-create');
+
+var $Object = Object;
+
+var initializer = function () {
+  var freeze = getBuiltIn('Object', 'freeze');
+  return freeze ? freeze(create(null)) : create(null);
+};
+
+// https://github.com/tc39/proposal-richer-keys/tree/master/compositeKey
+$({ global: true, forced: true }, {
+  compositeKey: function compositeKey() {
+    return apply(getCompositeKeyNode, $Object, arguments).get('object', initializer);
+  }
+});