Invite a new customer (staff-only) — emits a setup-token URL
post
/v1/auth/registerCreates a passwordless customer user, optionally creates / joins an org, issues a one-time setup-token, and returns the redemption URL. The invitee redeems via POST /v1/auth/setup. Gated by `artery_admin_session` — anyone with a staff session may invite; API keys cannot.