Implement Dead Letter Queue

# 🧠 Concept
Capture failed jobs for investigation.

## ⚠️ Problem
Repeated failures should not be silently discarded.

## 📁 Implementation Scope
- src/modules/queues/dlq/

## ✅ Requirements
- Failed job storage
- Retry controls
- Monitoring support

## 🎯 Acceptance Criteria
- [ ] DLQ implemented
- [ ] Failed jobs retained
- [ ] Recovery documented