feat: Add DEAD_LETTERED notification status for better failure tracking

[Emmyranky]

I have successfully implemented the requested notification dead-letter handling mechanism by introducing a new
DEAD_LETTERED status in the notification lifecycle.

Summary of Changes:

1. Updated NotificationStatus: Added DEAD_LETTERED to the NotificationStatus enum in
   listener/src/types/scheduled-notification.ts.
2. Updated ScheduledNotificationRepository:
   • Modified markAsFailedOrRetry to transition notifications to DEAD_LETTERED instead of FAILED when retries are
     exhausted.
   • Modified recoverStaleLocks to transition notifications to DEAD_LETTERED (and log the attempt correctly)
     instead of FAILED when a lock is recovered and retries are exhausted.
3. Updated NotificationExecutionLog: Added DEAD_LETTERED to the status type.
4. Updated dead-letter-queue.test.ts: Updated tests to verify that permanently failed notifications now correctly
   transition to DEAD_LETTERED status, and verified that requeuing functionality remains intact.

closes #706

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[Samuel-21-eng]

please review