perf: Pool 批量删除改为异步任务模式，避免大批量删除阻塞请求

- 新增 batch_delete_task 模块，提交删除后立即返回 task_id，后台线程分批执行
- 新增查询任务进度的 API 端点，前端轮询展示实时进度
- RequestCandidate 大表清理改为按行数分批删除，防止单条语句超时

frontend/src/api/endpoints/pool.ts

@@ -213,7 +213,7 @@ export async function listPoolKeys(
 export async function batchActionPoolKeys(
   providerId: string,
   body: PoolBatchAction,
-): Promise<{ affected: number; message: string }> {
+): Promise<{ affected: number; message: string; task_id?: string }> {
   const response = await client.post(
     `/api/admin/pool/${providerId}/keys/batch-action`,
     body,
@@ -222,6 +222,24 @@ export async function batchActionPoolKeys(
   return response.data
 }
 
+export interface BatchDeleteTaskStatus {
+  task_id: string
+  status: 'pending' | 'running' | 'completed' | 'failed'
+  total: number
+  deleted: number
+  message: string
+}
+
+export async function getPoolBatchDeleteTask(
+  providerId: string,
+  taskId: string,
+): Promise<BatchDeleteTaskStatus> {
+  const response = await client.get<BatchDeleteTaskStatus>(
+    `/api/admin/pool/${providerId}/keys/batch-delete-task/${taskId}`,
+  )
+  return response.data
+}
+
 export async function cleanupBannedPoolKeys(
   providerId: string,
 ): Promise<{ affected: number; message: string }> {