MailService renders transactional mails (verify-email, reset-password)
from the new de/en `mails` i18n namespace — text plus minimal HTML
with escaped interpolation — and enqueues them into mail_outbox.
MailWorker delivers pending rows every 15s through an injectable
transport (nodemailer; faked in tests) with quadratic backoff and a
permanent FAILED state after five attempts, logged as a warning.
SMTP_* and APP_BASE_URL join the environment schema with defaults
matching the new Mailpit container in the dev overlay.
Closes#12
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
