9e0f929e40
Add an integration test asserting the guest-until-confirmed squat fix (an email-login account is a reapable guest until the code is confirmed, then durable). Document the branded relay pipeline in ARCHITECTURE (go-mail, TLS by port, no client cert, PUBLIC_BASE_URL anti-injection, per-recipient send throttle, guest-until-confirmed), FUNCTIONAL (+ru), TESTING and the backend README config table (BACKEND_SMTP_* + BACKEND_PUBLIC_BASE_URL).