Registers a new attendee for the event. The payload is wrapped under the
attendee key.

Required fields: first_name, last_name, email, attendee_type_id.
(email may be skipped only when the selected attendee_type has
email_required disabled; default is required.)

If a draft attendee already exists for the same email + event it is
promoted; otherwise a new attendee is created and registered.

An optional top-level coupon_code is applied after a successful registration.

Precondition: registration must be enabled for the event; otherwise the request
fails with 422 "Registration for this event is not enabled".