List Undelivered Signups

Signups whose activation mail was never sent (admin).

mail_sent_at IS NULL on a request that was otherwise ready to go — every signup on a stack with SMTP_HOST unset, as alpha is today. From every other surface these people look like they lost interest.

A stamped mail_sent_at proves handoff to the relay, never delivery, so this catches the loud half of the failure and cannot see the quiet half (accepted by the relay, dropped downstream). Worth knowing before reading an empty list as "everybody got their mail".