[SMTP] Add reliable email delivery with retry and database-backed queue (#7240)

* [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

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/src/servatrice.h

@@ -21,6 +21,7 @@
 #define SERVATRICE_H
 
 #include "metrics_registry.h"
+#include "smtpclient.h"
 
 #include <QByteArray>
 #include <QDateTime>
@@ -181,6 +182,7 @@ private:
     QMap<QString, bool> serverRequiredFeatureList;
     QString officialWarnings;
     Servatrice_DatabaseInterface *servatriceDatabaseInterface;
+    bool cleanupDatabaseForEmail(const QString &userName, EmailType type, const QString &token, qint64 rowId);
     int serverId;
     int uptime;
     QMutex txBytesMutex, rxBytesMutex;
@@ -231,6 +233,12 @@ public slots:
     void scheduleShutdown(const QString &reason, int minutes);
     void updateLoginMessage();
     void setRequiredFeatures(const QString &featureList);
+    void onEmailDelivered(const QString &userName, EmailType type, const QString &token, qint64 rowId);
+    void onEmailPermanentlyFailed(const QString &userName,
+                                  EmailType type,
+                                  FailureReason reason,
+                                  const QString &token,
+                                  qint64 rowId);
 
 public:
     explicit Servatrice(QObject *parent = nullptr);