EventBus: dead-letter queue for poison messages

[adedw]

What to build

On BasicNack(requeue: true) a poison message loops forever. Add a dead-letter policy: declare email-sender with x-dead-letter-exchange + a separate DLQ; transient errors requeue, permanent failures dead-letter. Use the x-death header count to decide requeue vs dead-letter.

Acceptance criteria

• email-sender queue is declared with a dead-letter exchange and a dedicated DLQ.
• A permanently failing message dead-letters exactly once (no infinite loop), visible in the RabbitMQ management UI.
• Transient failures still requeue; requeue-vs-dead-letter decision uses x-death count.
• Sits on the connection/channel layer rebuilt in EventBus: auto-reconnect RabbitMQ consumer #81 — no conflict with the reconnect logic.

Blocked by

• EventBus: auto-reconnect RabbitMQ consumer #81