Idempotent notifications with an outbox: separate detection from sending, re-check before send

A Cloudflare Workers and D1 notification system uses the outbox pattern to deliver alerts without duplicates.

Detection writes only a deduplicated outbox row instead of sending immediately.

At send time, the system rechecks current data and decides to deliver, drop, or retry.

A state machine tracks pending, sending, sent, dropped, retry, and failed states.

The last_notified baseline is updated only after a successful send, preventing stale alerts.