📦 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/eachLimit.js

@@ -0,0 +1,50 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+
+var _eachOfLimit = require('./internal/eachOfLimit.js');
+
+var _eachOfLimit2 = _interopRequireDefault(_eachOfLimit);
+
+var _withoutIndex = require('./internal/withoutIndex.js');
+
+var _withoutIndex2 = _interopRequireDefault(_withoutIndex);
+
+var _wrapAsync = require('./internal/wrapAsync.js');
+
+var _wrapAsync2 = _interopRequireDefault(_wrapAsync);
+
+var _awaitify = require('./internal/awaitify.js');
+
+var _awaitify2 = _interopRequireDefault(_awaitify);
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+/**
+ * The same as [`each`]{@link module:Collections.each} but runs a maximum of `limit` async operations at a time.
+ *
+ * @name eachLimit
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.each]{@link module:Collections.each}
+ * @alias forEachLimit
+ * @category Collection
+ * @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
+ * @param {number} limit - The maximum number of async operations at a time.
+ * @param {AsyncFunction} iteratee - An async function to apply to each item in
+ * `coll`.
+ * The array index is not passed to the iteratee.
+ * If you need the index, use `eachOfLimit`.
+ * Invoked with (item, callback).
+ * @param {Function} [callback] - A callback which is called when all
+ * `iteratee` functions have finished, or an error occurs. Invoked with (err).
+ * @returns {Promise} a promise, if a callback is omitted
+ */
+function eachLimit(coll, limit, iteratee, callback) {
+    return (0, _eachOfLimit2.default)(limit)(coll, (0, _withoutIndex2.default)((0, _wrapAsync2.default)(iteratee)), callback);
+}
+exports.default = (0, _awaitify2.default)(eachLimit, 4);
+module.exports = exports.default;