📦 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/workbox-precaching/createHandlerBoundToURL.js

@@ -0,0 +1,31 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { getOrCreatePrecacheController } from './utils/getOrCreatePrecacheController.js';
+import './_version.js';
+/**
+ * Helper function that calls
+ * {@link PrecacheController#createHandlerBoundToURL} on the default
+ * {@link PrecacheController} instance.
+ *
+ * If you are creating your own {@link PrecacheController}, then call the
+ * {@link PrecacheController#createHandlerBoundToURL} on that instance,
+ * instead of using this function.
+ *
+ * @param {string} url The precached URL which will be used to lookup the
+ * `Response`.
+ * @param {boolean} [fallbackToNetwork=true] Whether to attempt to get the
+ * response from the network if there's a precache miss.
+ * @return {workbox-routing~handlerCallback}
+ *
+ * @memberof workbox-precaching
+ */
+function createHandlerBoundToURL(url) {
+    const precacheController = getOrCreatePrecacheController();
+    return precacheController.createHandlerBoundToURL(url);
+}
+export { createHandlerBoundToURL };