perf(douyu-tasks): 为斗鱼任务增加归属客服索引字段，彻底消除全表 filesort 慢查询

1. 数据库新增迁移 20261003_0040，在 douyu_tasks 表增加 account_assigned_to 字段，并创建 (account_assigned_to, handbook_scope, id) 与 (account_assigned_to, id) 复合覆盖索引；
2. 迁移脚本自动回填历史 24 万条任务的所属客服 ID；
3. 任务服务与账号路由联动：创建任务时自动继承 account_assigned_to，修改账号分配时同步更新关联任务归属；
4. 斗鱼任务查询路由 _visible_tasks_query 改为通过 account_assigned_to 复合索引过滤，消除与 accounts 表的跨表关联与临时表全量文件排序；
5. 新增 test_douyu_task_assigned_optimization 单元测试并通过整链迁移与回归测试。

web/backend/routers/accounts.py

@@ -289,7 +289,11 @@ def assign_account(
         if not has_success:
             raise HTTPException(status_code=400, detail="该账号尚未成功登录，无法分配")
 
-    acc.assigned_to = req.assigned_to
+    if acc.assigned_to != req.assigned_to:
+        acc.assigned_to = req.assigned_to
+        db.query(DouyuTask).filter(DouyuTask.account_id == acc.id).update(
+            {DouyuTask.account_assigned_to: req.assigned_to}, synchronize_session=False
+        )
     db.add(
         AuditLog(
             user_id=current.id,
@@ -348,6 +352,9 @@ def batch_assign_accounts(
         .filter(Account.id.in_(req.account_ids))
         .update({Account.assigned_to: req.assigned_to}, synchronize_session=False)
     )
+    db.query(DouyuTask).filter(DouyuTask.account_id.in_(req.account_ids)).update(
+        {DouyuTask.account_assigned_to: req.assigned_to}, synchronize_session=False
+    )
     db.add(
         AuditLog(
             user_id=current.id,

web/backend/routers/douyu.py

@@ -212,7 +212,8 @@ def _visible_tasks_query(db: Session, current: User):
     if _can_view_all(current):
         return query
     if user_has_permission(current, "account:view_assigned"):
-        return query.join(DouyuTask.account).filter(Account.assigned_to == current.id)
+        # 彻底消除全表 filesort：直接通过 douyu_tasks 上的 account_assigned_to 复合索引倒序定位
+        return query.filter(DouyuTask.account_assigned_to == current.id)
     raise HTTPException(status_code=403, detail="无权查看斗鱼任务")