Dead letter queue -- handle failed messages gracefully

Message queue consumers fail on certain messages (invalid format, missing data, transient errors). The message is redelivered, fails again, creating an infinite retry loop that blocks other messages.

After N failed processing attempts, move the message to a dead letter queue (DLQ). Benefits: (1) Unblock the main queue. (2) Preserve failed messages for investigation. (3) Enable manual or automated retry later. (4) Alert on DLQ growth. Setup: configure max delivery attempts (typically 3-5), set up DLQ as a separate queue, monitor DLQ depth, create tooling to inspect and replay DLQ messages.

Some messages are inherently unprocessable (corrupt data, deleted references). Without a DLQ, they block the queue forever or are silently dropped.