✨ Implement dead-letter queue (DLQ) enhancements by adding configuration options and validation logic. Update observability metrics to track DLQ message counts and publish failures. Enhance documentation to reflect new metrics and DLQ behavior in JetStream mode.

docs/reliability.md

@@ -15,6 +15,10 @@ subject = "caatsm.dlq"
 - When `dlq.enabled` is `true` and `dlq.subject` is non-empty, **permanent** failures are routed to the DLQ subject.  
 - A permanent failure is indicated by wrapping an error with `app.Permanent` and is treated as a **poison message**.
 
+> Note: DLQ routing only applies when the NATS mode is `jetstream`. In `core`
+> mode, the consumer does not attempt to publish to the DLQ even if it is
+> configured.
+
 Behaviour (JetStream mode):
 
 1. The NATS consumer calls `processor.Handle`.  
@@ -33,7 +37,8 @@ The DLQ subject should be consumed by an offline repair/analysis tool or operati
 
 - Inspect poison messages.  
 - Decide whether to fix and re-publish, or discard with justification.  
-- Track DLQ volume over time for alerting.
+- Track DLQ volume over time for alerting (e.g. via `caatsm_dlq_messages_total`
+  and `caatsm_dlq_publish_failures_total` metrics).
 
 ### Transient Errors and Backoff