📦 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/symbol-define-to-primitive.js

@@ -0,0 +1,21 @@
+'use strict';
+var call = require('../internals/function-call');
+var getBuiltIn = require('../internals/get-built-in');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+var defineBuiltIn = require('../internals/define-built-in');
+
+module.exports = function () {
+  var Symbol = getBuiltIn('Symbol');
+  var SymbolPrototype = Symbol && Symbol.prototype;
+  var valueOf = SymbolPrototype && SymbolPrototype.valueOf;
+  var TO_PRIMITIVE = wellKnownSymbol('toPrimitive');
+
+  if (SymbolPrototype && !SymbolPrototype[TO_PRIMITIVE]) {
+    // `Symbol.prototype[@@toPrimitive]` method
+    // https://tc39.es/ecma262/#sec-symbol.prototype-@@toprimitive
+    // eslint-disable-next-line no-unused-vars -- required for .length
+    defineBuiltIn(SymbolPrototype, TO_PRIMITIVE, function (hint) {
+      return call(valueOf, this);
+    }, { arity: 1 });
+  }
+};