In Catalyst, can I use transactional outbox patterns to achieve exactly-once side effects with databases?

Yes, you can implement a transactional outbox pattern within Catalyst's workflow activities, where side effects are written to a database in the same transaction as workflow state. Durable execution does not make delivery exactly-once — it is at-least-once, and a step may run again. What prevents a duplicate side effect is the database transaction plus an idempotency key on the consumer, so the property comes from the outbox pattern rather than from durable execution.