[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.

servatrice/servatrice.ini.example

@@ -244,6 +244,16 @@ email=root@localhost
 ; Sender email name
 name="Cockatrice server"
 
+; Maximum number of attempts to deliver a given e-mail before giving up.
+; If the smtp server is unreachable or repeatedly rejects a message, the server
+; will retry with an exponentially growing delay up to this many times, then drop
+; the message from the queue and log the failure. Default: 5
+maxretries=5
+
+; Initial retry delay in seconds between delivery attempts. Doubles with each
+; failed attempt (capped at 30 minutes). Default: 60
+retrydelay=60
+
 ; Email subject
 subject="Cockatrice server account activation token"