feat(reliability): dead-letter queue for poison events

[MarkCesium]

Summary

FastStream default behaviour on consumer exception is redelivery forever. A single malformed event halts progress on that partition.

Changes

• Consumer wrapper: N retries with exponential backoff (default 5, 1s → 30s).
• After N failures, publish the original message to devices.dlq with headers original_topic, error, traceback.
• Prometheus counter devices_dlq_total{topic} and gauge devices_dlq_depth (see feat(observability): structured logging, metrics, tracing #19).

Verification

• Feed a malformed message to places.member.added — after 5 retries it lands on devices.dlq, main topic progresses, metric increments. Alert from infra#19 fires.