[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/smtpclient.h

@@ -1,26 +1,67 @@
 #ifndef SMTPCLIENT_H
 #define SMTPCLIENT_H
 
+#include "smtp/qxtmailmessage.h"
+
+#include <QDateTime>
+#include <QHash>
 #include <QObject>
 #include <QString>
+#include <QTimer>
 #include <qtmetamacros.h>
 
 class QxtSmtp;
 class QByteArray;
 
+/**
+ * @brief Types of e-mail the SMTP client is able to deliver.
+ */
+enum class EmailType
+{
+    Activation,
+    ForgotPassword
+};
+
+Q_DECLARE_METATYPE(EmailType)
+
+/**
+ * @brief Why a permanent e-mail delivery failure occurred.
+ */
+enum class FailureReason
+{
+    RetryExhausted,
+    RecipientRejected,
+    SenderRejected
+};
+
+Q_DECLARE_METATYPE(FailureReason)
+
 class SmtpClient : public QObject
 {
     Q_OBJECT
 public:
-    SmtpClient(QObject *parent = 0);
-    ~SmtpClient();
+    SmtpClient(QObject *parent = nullptr);
+    ~SmtpClient() override;
+
+public slots:
+    bool enqueueActivationTokenMail(const QString &nickname, const QString &recipient, const QString &token);
+    bool enqueueForgotPasswordTokenMail(const QString &nickname,
+                                        const QString &recipient,
+                                        const QString &token,
+                                        qint64 rowId = -1);
+    void sendAllEmails();
+
+signals:
+    void mailDelivered(const QString &userName, EmailType type, const QString &token, qint64 rowId);
+    void mailPermanentlyFailed(const QString &userName,
+                               EmailType type,
+                               FailureReason reason,
+                               const QString &token,
+                               qint64 rowId);
 
 protected:
     QxtSmtp *smtp;
-public slots:
-    bool enqueueActivationTokenMail(const QString &nickname, const QString &recipient, const QString &token);
-    bool enqueueForgotPasswordTokenMail(const QString &nickname, const QString &recipient, const QString &token);
-    void sendAllEmails();
+
 protected slots:
     void authenticated();
     void authenticationFailed(const QByteArray &msg);
@@ -30,10 +71,55 @@ protected slots:
     void encrypted();
     void encryptionFailed(const QByteArray &msg);
     void finished();
+    void idleTimeout();
     void mailFailed(int mailID, int errorCode, const QByteArray &msg);
     void mailSent(int mailID);
     void recipientRejected(int mailID, const QString &address, const QByteArray &msg);
     void senderRejected(int mailID, const QString &address, const QByteArray &msg);
+
+private:
+    struct PendingEmail
+    {
+        QString userName;
+        EmailType type;
+        QString token;
+        // Database row id for tables that allow multiple rows per user
+        // ({prefix}_forgot_password); -1 when the name alone identifies the row.
+        qint64 rowId = -1;
+        QxtMailMessage message;
+        int attempts = 0;
+        int maxAttempts = 0;
+        int retryDelay = 0;
+        QDateTime nextAttempt;
+    };
+
+    PendingEmail buildEmail(const QString &nickname,
+                            EmailType type,
+                            const QString &subject,
+                            const QString &body,
+                            const QString &recipient,
+                            const QString &token,
+                            const QString &email,
+                            qint64 rowId = -1);
+    bool enqueue(const PendingEmail &email);
+    void relayoutForSending();
+    QDateTime nextRetryTime(int attempts, int retryDelay) const;
+    void recordFailed(int mailID);
+    void recordConnectionFailure();
+    bool validateEmailFields(const QString &email,
+                             const QString &subject,
+                             const QString &body,
+                             const QString &recipient,
+                             const QString &token,
+                             const QString &nickname);
+    int maxRetries() const;
+    int retryDelaySeconds() const;
+
+    QHash<QString, PendingEmail> pendingEmails;
+    QHash<int, QString> mailIdToUser;
+    QTimer *idleTimer;
+    int connectionAttempts = 0;
+    QDateTime connectionBackoffUntil;
 };
 
-#endif
+#endif