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/models.py

@@ -8,6 +8,7 @@ from sqlalchemy import (
     JSON,
     Boolean,
     ForeignKey,
+    Index,
     Integer,
     String,
     Text,
@@ -214,6 +215,24 @@ class DouyuTask(Base):
     created_by: Mapped[int] = mapped_column(ForeignKey("users.id"), nullable=False)
     created_at: Mapped[datetime] = mapped_column(UTCDateTime, default=_utcnow)
     finished_at: Mapped[datetime | None] = mapped_column(UTCDateTime, nullable=True)
+    # 账号归属客服冗余字段，配合复合索引消除任务列表全表 filesort。
+    account_assigned_to: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id"), nullable=True
+    )
+
+    __table_args__ = (
+        Index(
+            "ix_douyu_tasks_account_assigned_to_id",
+            "account_assigned_to",
+            "id",
+        ),
+        Index(
+            "ix_douyu_tasks_assigned_scope_id",
+            "account_assigned_to",
+            "handbook_scope",
+            "id",
+        ),
+    )
 
     account: Mapped[Account] = relationship("Account", back_populates="douyu_tasks")