📦 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.set.intersection.js

@@ -0,0 +1,14 @@
+'use strict';
+var $ = require('../internals/export');
+var call = require('../internals/function-call');
+var toSetLike = require('../internals/to-set-like');
+var $intersection = require('../internals/set-intersection');
+
+// `Set.prototype.intersection` method
+// https://github.com/tc39/proposal-set-methods
+// TODO: Obsolete version, remove from `core-js@4`
+$({ target: 'Set', proto: true, real: true, forced: true }, {
+  intersection: function intersection(other) {
+    return call($intersection, this, toSetLike(other));
+  }
+});