📦 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/dependencies/NullDependency.js

@@ -0,0 +1,42 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Tobias Koppers @sokra
+*/
+
+"use strict";
+
+const Dependency = require("../Dependency");
+const DependencyTemplate = require("../DependencyTemplate");
+
+/** @typedef {import("webpack-sources").ReplaceSource} ReplaceSource */
+/** @typedef {import("../Dependency").TRANSITIVE} TRANSITIVE */
+/** @typedef {import("../DependencyTemplate").DependencyTemplateContext} DependencyTemplateContext */
+
+/** @typedef {string[]} RawRuntimeRequirements */
+
+class NullDependency extends Dependency {
+	get type() {
+		return "null";
+	}
+
+	/**
+	 * @returns {boolean | TRANSITIVE} true, when changes to the referenced module could affect the referencing module; TRANSITIVE, when changes to the referenced module could affect referencing modules of the referencing module
+	 */
+	couldAffectReferencingModule() {
+		return false;
+	}
+}
+
+NullDependency.Template = class NullDependencyTemplate extends (
+	DependencyTemplate
+) {
+	/**
+	 * @param {Dependency} dependency the dependency for which the template should be applied
+	 * @param {ReplaceSource} source the current replace source which can be modified
+	 * @param {DependencyTemplateContext} templateContext the context object
+	 * @returns {void}
+	 */
+	apply(dependency, source, templateContext) {}
+};
+
+module.exports = NullDependency;