Add retention policies + cleanup jobs for auth_nonces, DLQ, processed events

[robertocarlous]

Problem

Tables like auth_nonces, processed_events, dead_letter_events, and agent_logs can grow without bound. This degrades performance and increases costs.

Acceptance criteria

• Add scheduled cleanup jobs:
  • delete expired auth_nonces
  • archive or prune old processed_events (configurable retention)
  • retention strategy for agent_logs (configurable)
• Add indexes where required for cleanup queries
• Add metrics: rows deleted, job duration, last successful run
• Document retention defaults and how to override via env

model AuthNonce { ... expiresAt ... }

Priority

P1 / High