feat: 新增后台任务系统，支持抓取任务在后台持续运行

- 新增后台任务API (/api/tasks)
- 抓取任务在后台独立运行，不受页面刷新影响
- 支持任务状态查询和手动停止
- 新增后台任务管理界面
- 数据库新增 background_tasks 表
- 前端使用轮询方式更新任务进度

static/css/search.css

@@ -454,4 +454,120 @@
 .failed-url-actions {
     display: flex;
     gap: 8px;
+}
+
+/* 后台任务区域 */
+.background-tasks-section {
+    background: white;
+    border-radius: 12px;
+    box-shadow: 0 2px 4px rgba(0,0,0,0.05);
+    margin-top: 20px;
+}
+
+.background-tasks-section .panel-header {
+    background: #e0e7ff;
+}
+
+.background-tasks-section .panel-header h2 {
+    color: #3730a3;
+}
+
+.background-tasks-list {
+    display: flex;
+    flex-direction: column;
+    gap: 10px;
+}
+
+.background-task-item {
+    display: flex;
+    justify-content: space-between;
+    align-items: flex-start;
+    padding: 15px;
+    border: 1px solid #c7d2fe;
+    border-radius: 8px;
+    background: #f5f3ff;
+}
+
+.background-task-item.status-running {
+    background: #fef3c7;
+    border-color: #fcd34d;
+}
+
+.background-task-item.status-success {
+    background: #d1fae5;
+    border-color: #6ee7b7;
+}
+
+.background-task-item.status-error {
+    background: #fee2e2;
+    border-color: #fca5a5;
+}
+
+.background-task-item.status-warning {
+    background: #fef3c7;
+    border-color: #fcd34d;
+}
+
+.task-info {
+    flex: 1;
+}
+
+.task-id {
+    font-size: 12px;
+    color: #666;
+    margin-bottom: 5px;
+}
+
+.task-status {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    margin-bottom: 5px;
+}
+
+.task-progress {
+    color: #667eea;
+    font-weight: bold;
+}
+
+.status-badge {
+    padding: 2px 8px;
+    border-radius: 4px;
+    font-size: 12px;
+    font-weight: 500;
+}
+
+.status-badge.status-pending {
+    background: #e0e7ff;
+    color: #3730a3;
+}
+
+.status-badge.status-running {
+    background: #fef3c7;
+    color: #92400e;
+}
+
+.status-badge.status-success {
+    background: #d1fae5;
+    color: #065f46;
+}
+
+.status-badge.status-error {
+    background: #fee2e2;
+    color: #991b1b;
+}
+
+.status-badge.status-warning {
+    background: #fef3c7;
+    color: #92400e;
+}
+
+.task-time {
+    font-size: 11px;
+    color: #999;
+}
+
+.task-actions {
+    display: flex;
+    gap: 8px;
 }