Register an end user ({"end_user": "<user or safety_identifier>"}) as no-train: from now on that user’s
requests are answered as usual but never stored (FR-13.2), whether a request names the user in user or in
safety_identifier. Only the HMAC is kept, and it is not returned: the endpoint must not become a way to compute
hashes. Open where chat completions are (an opt-out only removes storage); an admin may name a client, or
register the user for every client, also those added later (the every-client scope). Whether the end user was
registered before (created) is told to an API key or an admin key only: without keys anyone may call this.