血缘锚定法: task_type字段+迁移+三大接口写入+TreeCanvas基于task_type布局

backend/app/schemas/task.py

@@ -131,6 +131,7 @@ class TaskResponse(BaseModel):
     status: str
     notify_parent_on_complete: bool
     is_rework: bool = False
+    task_type: str | None = None
     remark: str | None = None
     reject_reason: str | None = None
     received_at: datetime | None = None