Delete OAuth client

Permanently delete an OAuth client. This cannot be undone.

Terminal, unlike the reversible kill switch (DELETE on this resource, which only sets active=false): every active grant is revoked, every token carrying the client's lineage is revoked, and the registration row is removed — connected applications are fully disconnected. The audit trail survives. A client that later re-registers via dynamic client registration is a NEW registration and re-enters the approval queue as pending; it is never re-attached to the deleted one.