Adds soft‑deletion support for workflows through a new boolean column `isArchived`.
When a workflow is archived we now set `isArchived` flag to true and the workflows
stays in the database and is omitted from the default workflow listing query.
Archived workflows can be viewed in read-only mode, but they cannot be activated.
Archived workflows are still available by ID and can be invoked as sub-executions,
so existing Execute Workflow nodes continue to work. Execution engine doesn't
care about isArchived flag.
Users can restore workflows via Unarchive action at the UI.
