📦 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.reflect.define-metadata.js

@@ -0,0 +1,17 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var $ = require('../internals/export');
+var ReflectMetadataModule = require('../internals/reflect-metadata');
+var anObject = require('../internals/an-object');
+
+var toMetadataKey = ReflectMetadataModule.toKey;
+var ordinaryDefineOwnMetadata = ReflectMetadataModule.set;
+
+// `Reflect.defineMetadata` method
+// https://github.com/rbuckton/reflect-metadata
+$({ target: 'Reflect', stat: true }, {
+  defineMetadata: function defineMetadata(metadataKey, metadataValue, target /* , targetKey */) {
+    var targetKey = arguments.length < 4 ? undefined : toMetadataKey(arguments[3]);
+    ordinaryDefineOwnMetadata(metadataKey, metadataValue, anObject(target), targetKey);
+  }
+});