📦 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/errors/BuildCycleError.js

@@ -0,0 +1,30 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Tobias Koppers @sokra
+*/
+
+"use strict";
+
+const WebpackError = require("../WebpackError");
+
+/** @typedef {import("../Module")} Module */
+
+class BuildCycleError extends WebpackError {
+	/**
+	 * Creates an instance of BuildCycleError.
+	 * @param {Module} module the module starting the cycle
+	 */
+	constructor(module) {
+		super(
+			"There is a circular build dependency, which makes it impossible to create this module"
+		);
+
+		/** @type {string} */
+		this.name = "BuildCycleError";
+		/** @type {Module} */
+		this.module = module;
+	}
+}
+
+/** @type {typeof BuildCycleError} */
+module.exports = BuildCycleError;