Permanently delete an archived BOM

Starts an asynchronous permanent delete of an archived BOM. The BOM is moved to status 'deleting' immediately and returns 202; a background worker then removes its lines, comparisons, and the BOM row, and deletes the items it created that are safe to remove (no documents attached, not ERP-linked, not referenced by another active BOM). This cannot be undone. The BOM must be 'archived'; a BOM already 'deleting' returns 409.