Restore an archived dashboard (org admin only).

Brings an archived dashboard back. Requires share:manage. 404 when the id is not an archived dashboard of the caller's org. Because archive shares the soft-delete gate, this also restores a dashboard its owner deleted; the two are told apart in the audit log (dashboard.archive vs delete). Unarchive brings back the dashboard's previous visibility, ACL grants, unexpired embed tokens, scheduled exports and email reports, whether it was archived or deleted. A dashboard whose folder was deleted in the meantime comes back unfiled. Audit-logged as dashboard.unarchive. Not the same as POST /v1/dashboards/{id}/restore, which rolls a live dashboard back to an earlier revision.