When Kubernetes performs rolling deployments, the old pod gracefully shuts down
and Oban's internal processes receive EXIT :shutdown signals. These are logged
as "unexpected" even though they're normal during shutdown.
Added custom logger filter to drop these messages in production:
- Oban.Queue.Watchman/Producer receiving EXIT :shutdown
- Oban plugin processes receiving EXIT :shutdown
- Reduces log noise during deployments without hiding real errors
The filter is production-only to preserve full debugging in development.
