重试失败任务时复用旧记录而非创建新记录

- 重试时查找该账号最近一条失败任务，更新其状态为 pending
- 避免同一账号在列表中出现多条重复记录
- 仅复用 failed/error 状态的任务，首次登录仍新建记录

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/backend/services/login_service.py

@@ -156,7 +156,7 @@ class LoginBatchRunner:
         concurrency = self.concurrency
         self._push_log("info", f"批量登录任务 {batch_id} 开始，共 {len(self.account_ids)} 个账号，并发数: {concurrency}")
 
-        # 创建任务记录（顺序执行，线程安全）
+        # 创建或复用任务记录（顺序执行，线程安全）
         task_infos: list[dict] = []  # {task_id, acc_info}
         for aid in self.account_ids:
             acc = self.db.query(AccountModel).filter(AccountModel.id == aid).first()
@@ -168,13 +168,29 @@ class LoginBatchRunner:
                     self._push_log("warning", f"跳过无权账号: {acc.username}")
                     continue
 
-            task = LoginTask(
-                batch_id=batch_id,
-                account_id=aid,
-                status="pending",
-                created_by=self.created_by,
+            # 复用该账号最近一条失败任务记录，避免重复产生多条
+            existing_task = (
+                self.db.query(LoginTask)
+                .filter(LoginTask.account_id == aid, LoginTask.status.in_(["failed", "error"]))
+                .order_by(LoginTask.id.desc())
+                .first()
             )
-            self.db.add(task)
+            if existing_task:
+                existing_task.batch_id = batch_id
+                existing_task.status = "pending"
+                existing_task.cookie = ""
+                existing_task.message = ""
+                existing_task.finished_at = None
+                task = existing_task
+            else:
+                task = LoginTask(
+                    batch_id=batch_id,
+                    account_id=aid,
+                    status="pending",
+                    created_by=self.created_by,
+                )
+                self.db.add(task)
+
             self.db.flush()  # 获取 task.id
 
             task_infos.append({