📦 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/async/internal/awaitify.js

@@ -0,0 +1,28 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+exports.default = awaitify;
+// conditionally promisify a function.
+// only return a promise if a callback is omitted
+function awaitify(asyncFn, arity) {
+    if (!arity) arity = asyncFn.length;
+    if (!arity) throw new Error('arity is undefined');
+    function awaitable(...args) {
+        if (typeof args[arity - 1] === 'function') {
+            return asyncFn.apply(this, args);
+        }
+
+        return new Promise((resolve, reject) => {
+            args[arity - 1] = (err, ...cbArgs) => {
+                if (err) return reject(err);
+                resolve(cbArgs.length > 1 ? cbArgs : cbArgs[0]);
+            };
+            asyncFn.apply(this, args);
+        });
+    }
+
+    return awaitable;
+}
+module.exports = exports.default;