perf: 依赖数据库 CASCADE/SET NULL 替代手动清理关联表，缩短删除事务

- 批量删除移除 cleanup_key_references 手动清理，改为依赖 FK CASCADE/SET NULL
- video_tasks.key_id FK 增加 ondelete="SET NULL"，附带幂等迁移脚本
- _sync_delete 增加 statement_timeout 和任务级超时保护
- 批量导入在每次 await 前释放闲置 DB 连接，按批次提交写入避免长事务
- 前端轮询改为先查后等，首次查询不再多等一个间隔

frontend/src/features/pool/components/PoolAccountBatchDialog.vue

@@ -585,7 +585,6 @@ async function pollDeleteTask(
   const deadline = Date.now() + DELETE_POLL_MAX_MS
   let consecutiveFailures = 0
   while (Date.now() < deadline) {
-    await new Promise((r) => setTimeout(r, DELETE_POLL_INTERVAL_MS))
     try {
       const task = await getPoolBatchDeleteTask(providerId, taskId)
       consecutiveFailures = 0
@@ -599,6 +598,7 @@ async function pollDeleteTask(
         return { status: 'failed', deleted: 0 }
       }
     }
+    await new Promise((r) => setTimeout(r, DELETE_POLL_INTERVAL_MS))
   }
   return { status: 'failed', deleted: 0 }
 }