fix: schedule_delete 外键约束导致无法删除定时任务

- schedule_delete_tool 执行前先解除 executions.schedule_id 引用
- 迁移 012: executions.schedule_id FK 改为 ON DELETE SET NULL

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/012_schedule_fk_set_null.py

@@ -0,0 +1,43 @@
+"""alter executions.schedule_id FK to ON DELETE SET NULL
+
+Revision ID: 012_schedule_fk_set_null
+Revises: 011_add_agent_market_fields
+Create Date: 2026-05-07
+"""
+from alembic import op
+
+revision = "012_schedule_fk_set_null"
+down_revision = "011_add_agent_market_fields"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """将 executions.schedule_id 外键改为 SET NULL，删除定时任务时自动保留执行记录。"""
+    try:
+        op.drop_constraint("executions_ibfk_3", "executions", type_="foreignkey")
+    except Exception:
+        # FK name may vary; try alternate patterns
+        try:
+            op.drop_constraint("executions_ibfk_4", "executions", type_="foreignkey")
+        except Exception:
+            pass
+    op.create_foreign_key(
+        "executions_ibfk_3",
+        "executions", "agent_schedules",
+        ["schedule_id"], ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade() -> None:
+    try:
+        op.drop_constraint("executions_ibfk_3", "executions", type_="foreignkey")
+    except Exception:
+        pass
+    op.create_foreign_key(
+        "executions_ibfk_3",
+        "executions", "agent_schedules",
+        ["schedule_id"], ["id"],
+        ondelete="RESTRICT",
+    )