📦 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-background-sync/BackgroundSyncPlugin.js

@@ -0,0 +1,36 @@
+/*
+  Copyright 2018 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 { Queue } from './Queue.js';
+import './_version.js';
+/**
+ * A class implementing the `fetchDidFail` lifecycle callback. This makes it
+ * easier to add failed requests to a background sync Queue.
+ *
+ * @memberof workbox-background-sync
+ */
+class BackgroundSyncPlugin {
+    /**
+     * @param {string} name See the {@link workbox-background-sync.Queue}
+     *     documentation for parameter details.
+     * @param {Object} [options] See the
+     *     {@link workbox-background-sync.Queue} documentation for
+     *     parameter details.
+     */
+    constructor(name, options) {
+        /**
+         * @param {Object} options
+         * @param {Request} options.request
+         * @private
+         */
+        this.fetchDidFail = async ({ request }) => {
+            await this._queue.pushRequest({ request });
+        };
+        this._queue = new Queue(name, options);
+    }
+}
+export { BackgroundSyncPlugin };