📦 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/es.object.get-own-property-symbols.js

@@ -0,0 +1,19 @@
+'use strict';
+var $ = require('../internals/export');
+var NATIVE_SYMBOL = require('../internals/symbol-constructor-detection');
+var fails = require('../internals/fails');
+var getOwnPropertySymbolsModule = require('../internals/object-get-own-property-symbols');
+var toObject = require('../internals/to-object');
+
+// V8 ~ Chrome 38 and 39 `Object.getOwnPropertySymbols` fails on primitives
+// https://bugs.chromium.org/p/v8/issues/detail?id=3443
+var FORCED = !NATIVE_SYMBOL || fails(function () { getOwnPropertySymbolsModule.f(1); });
+
+// `Object.getOwnPropertySymbols` method
+// https://tc39.es/ecma262/#sec-object.getownpropertysymbols
+$({ target: 'Object', stat: true, forced: FORCED }, {
+  getOwnPropertySymbols: function getOwnPropertySymbols(it) {
+    var $getOwnPropertySymbols = getOwnPropertySymbolsModule.f;
+    return $getOwnPropertySymbols ? $getOwnPropertySymbols(toObject(it)) : [];
+  }
+});