📦 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/modules/esnext.map.key-by.js

@@ -0,0 +1,22 @@
+'use strict';
+var $ = require('../internals/export');
+var call = require('../internals/function-call');
+var iterate = require('../internals/iterate');
+var isCallable = require('../internals/is-callable');
+var aCallable = require('../internals/a-callable');
+var Map = require('../internals/map-helpers').Map;
+
+// `Map.keyBy` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'Map', stat: true, forced: true }, {
+  keyBy: function keyBy(iterable, keyDerivative) {
+    var C = isCallable(this) ? this : Map;
+    var newMap = new C();
+    aCallable(keyDerivative);
+    var setter = aCallable(newMap.set);
+    iterate(iterable, function (element) {
+      call(setter, newMap, keyDerivative(element), element);
+    });
+    return newMap;
+  }
+});