Send (or resend) a magic-link email that lets the traveler create an account while their selection stays held. Requires contact email, both consents, and a locked selection on the session. The link expires when the selection hold does (magicLinkExpiresAt equals selectionHoldExpiresAt), and the email states that time. A resend invalidates every previously emailed link. The link always goes to the session's contact email, which cannot be changed once the first link has been sent.