📦 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/es.date.get-year.js

@@ -0,0 +1,20 @@
+'use strict';
+var $ = require('../internals/export');
+var uncurryThis = require('../internals/function-uncurry-this');
+var fails = require('../internals/fails');
+
+// IE8- non-standard case
+var FORCED = fails(function () {
+  // eslint-disable-next-line es/no-date-prototype-getyear-setyear -- detection
+  return new Date(16e11).getYear() !== 120;
+});
+
+var getFullYear = uncurryThis(Date.prototype.getFullYear);
+
+// `Date.prototype.getYear` method
+// https://tc39.es/ecma262/#sec-date.prototype.getyear
+$({ target: 'Date', proto: true, forced: FORCED }, {
+  getYear: function getYear() {
+    return getFullYear(this) - 1900;
+  }
+});