feat(api): Add `node_execution_id` column to `WorkflowDraftVariable`

Introduce a `node_execution_id` column to the `WorkflowDraftVariable` table
to enable efficient implementation of the "reset to last run value" functionality.

This optimization avoids querying the potentially large `workflow_node_executions`
table, which is particularly beneficial for scaling in our SaaS environment.

api/models/workflow.py

@@ -892,6 +892,7 @@ class WorkflowDraftVariable(Base):
     # visible
     visible: Mapped[bool] = mapped_column(sa.Boolean, nullable=False, default=True)
     editable: Mapped[bool] = mapped_column(sa.Boolean, nullable=False, default=False)
+    node_execution_id: Mapped[str] = mapped_column(StringUUID, nullable=False)
 
     def get_selector(self) -> list[str]:
         selector = json.loads(self.selector)