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/alembic/versions/a7b8c9d0e1f2_add_task_type.py

@@ -0,0 +1,24 @@
+"""add_task_type
+
+Revision ID: a7b8c9d0e1f2
+Revises: f6a7b8c9d0e1
+Create Date: 2026-08-06
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "a7b8c9d0e1f2"
+down_revision: Union[str, None] = "f6a7b8c9d0e1"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("tasks", sa.Column("task_type", sa.String(20), nullable=True, comment="派生类型: TRANSFER/SPAWN/RECOVERY"))
+    # 刷老数据：parent_task_id 非空的默认为 TRANSFER
+    op.execute("UPDATE tasks SET task_type = 'TRANSFER' WHERE parent_task_id IS NOT NULL AND task_type IS NULL")
+
+
+def downgrade() -> None:
+    op.drop_column("tasks", "task_type")