Persist delivery queue across restarts #9

Summary

Back the delivery retry queue with durable storage so pending deliveries survive restarts.

Motivation

The current in-memory queue loses outstanding deliveries on process exit. Persisting queue state would make retry logic resilient to crashes and planned restarts.

Proposed direction

• Store queue state on disk (e.g., in spool directory) or use a lightweight embedded database.
• Ensure enqueue/dequeue flows remain efficient and do not block the main loop.
• Provide migration/upgrade path for existing deployments.

Labels

• feature
• queue