📦 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/internals/new-promise-capability.js

@@ -0,0 +1,21 @@
+'use strict';
+var aCallable = require('../internals/a-callable');
+
+var $TypeError = TypeError;
+
+var PromiseCapability = function (C) {
+  var resolve, reject;
+  this.promise = new C(function ($$resolve, $$reject) {
+    if (resolve !== undefined || reject !== undefined) throw new $TypeError('Bad Promise constructor');
+    resolve = $$resolve;
+    reject = $$reject;
+  });
+  this.resolve = aCallable(resolve);
+  this.reject = aCallable(reject);
+};
+
+// `NewPromiseCapability` abstract operation
+// https://tc39.es/ecma262/#sec-newpromisecapability
+module.exports.f = function (C) {
+  return new PromiseCapability(C);
+};