feat(dlq): add dead-letter isolation and operator remediation system (#706)

[ravendevhub]

Summary

Resolves #706 by establishing a dedicated Dead Letter Queue (DLQ) manager (listener/src/services/dead-letter-manager.ts) that isolates repeatedly failing or exhausted notifications to prevent head-of-line blocking in the dispatch pipeline while preserving full failure traceability and operator remediation capabilities.

Changes

1. Dead Letter Manager (listener/src/services/dead-letter-manager.ts):
   • Isolates exhausted notification payloads into structured DLQ entries.
   • Retains original notification ID, provider transport, attempt count, and sanitized error stack traces.
   • Exposes operator methods: listDeadLetters(), getDeadLetter(), requeue(), discard(), and getMetrics().
2. Automated Unit Tests (listener/src/services/dead-letter-manager.test.ts):
   • Tests isolation with redaction, listing with filters, requeuing lifecycle, and queue metrics.
3. Documentation (docs/DEAD_LETTER_HANDLING.md):
   • Schema specification and operator remediation procedures.

Acceptance Criteria

• Repeatedly failed notifications can be isolated.
• Dead-lettered notifications remain traceable.
• Operators can inspect failure reasons.
• Normal notification processing continues.