[Fresh 2026-04][Backend] Webhooks: DLQ + authenticated replay endpoint with audit logging

[Baskarayelu]

Description

Add DLQ for failed webhook deliveries and a replay endpoint that is audited and idempotent.

Requirements and context

• Backend-only; rate-limited; tested.

Suggested execution

• Branch: feature/webhook-dlq-replay-fresh
• Implement: src/jobs/, src/routes/, src/repositories/
• Tests: tests/integration/

Example commit message

feat(webhooks): add DLQ and audited replay endpoint

Guidelines

• Timeframe: 96 hours