* [SMTP] Add reliable email delivery with retry and database-backed queue
Rework the SMTP client from fire-and-forget sending into a persistent
queue with exponential backoff and per-message retry budget, backed by
the existing database rows as the durable source of truth.
- Keep pending emails in an in-memory queue with attempts/backoff
tracking; enqueue without deleting database rows, and only remove the
row once delivery is confirmed via the mailDelivered signal.
- Add exponential backoff (doubling, capped at 30 min) between delivery
attempts, bounded by new smtp/maxretries and smtp/retrydelay settings.
- Track connection-layer failures (connect/auth/encrypt) with their own
backoff; route failures to recordFailed/recordConnectionFailure so
undeliverable mail eventually drops from the queue instead of
retrying forever.
- Treat rejected recipients/senders as permanent failures.
- Rebuild the SMTP buffer from the authoritative queue before each send
via a new QxtSmtp::reset() so reconnects never duplicate deliveries.
- Start the idle timeout after authentication, not on TCP connect, so
slow SSL/STARTTLS handshakes are not cut short.
* [SMTP] Fix retry accounting, backoff reset, and token dedupe
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Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>
