Send Test Alert

Send a synthetic alert email so SMTP and recipient wiring can be verified.

Worth having because the alert path is, by design, exercised only when something is already broken. Discovering then that ALERT_EMAIL had a typo is the worst possible time.

On failure the transport's own explanation is returned rather than a generic "not sent", which previously sent the operator to the container logs to learn whether the cause was a typo, a rejected password, or a blocked port.