Design an idempotent consumer for Apaleo's at-least-once webhook delivery

Steps

1. Assign or extract a stable event/message identifier from each inbound Apaleo webhook payload.
2. Persist processed event IDs (e.g., a dedup table with TTL) before applying side effects, so a redelivered event is a no-op.
3. Return a fast 2xx acknowledgment immediately and defer slow work (folio updates, external syncs) to an async queue.
4. Handle out-of-order delivery by applying updates based on a version/timestamp field rather than blindly overwriting local state.
5. Add alerting for endpoints that fall behind, since the roughly one-minute retry cadence can pile up during an outage.