📦 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/webpack/lib/util/AppendOnlyStackedSet.js

@@ -0,0 +1,78 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Ivan Kopeykin @vankop
+*/
+
+"use strict";
+
+/**
+ * @template T
+ */
+class AppendOnlyStackedSet {
+	/**
+	 * @param {Set<T>[]} sets an optional array of sets
+	 */
+	constructor(sets = []) {
+		/** @type {Set<T>[]} */
+		this._sets = sets;
+		/** @type {Set<T> | undefined} */
+		this._current = undefined;
+	}
+
+	/**
+	 * @param {T} el element
+	 */
+	add(el) {
+		if (!this._current) {
+			this._current = new Set();
+			this._sets.push(this._current);
+		}
+		this._current.add(el);
+	}
+
+	/**
+	 * @param {T} el element
+	 * @returns {boolean} result
+	 */
+	has(el) {
+		for (const set of this._sets) {
+			if (set.has(el)) return true;
+		}
+		return false;
+	}
+
+	clear() {
+		this._sets = [];
+		if (this._current) {
+			this._current = undefined;
+		}
+	}
+
+	/**
+	 * @returns {AppendOnlyStackedSet<T>} child
+	 */
+	createChild() {
+		return new AppendOnlyStackedSet(this._sets.length ? [...this._sets] : []);
+	}
+
+	/**
+	 * @returns {Iterator<T>} iterable iterator
+	 */
+	[Symbol.iterator]() {
+		const iterators = this._sets.map((map) => map[Symbol.iterator]());
+		let current = iterators.pop();
+		return {
+			next() {
+				if (!current) return { done: true, value: undefined };
+				let result = current.next();
+				while (result.done && iterators.length > 0) {
+					current = /** @type {SetIterator<T>} */ (iterators.pop());
+					result = current.next();
+				}
+				return result;
+			}
+		};
+	}
+}
+
+module.exports = AppendOnlyStackedSet;