Idempotency, retries and delivery order

Sending: always send an Idempotency-Key (UUID) on POST /v1/messages; a retry with the same key returns the same record (200) instead of duplicating. The source of truth is MariaDB (we persist before enqueuing), so a 202 is never lost even if Redis goes down.

Delivery (webhooks): retries with backoff 1m → 5m → 30m → 2h → 6h (5 attempts). The X-Connect-Delivery-Id is stable across retries → deduplicate on it. After 20 consecutive failures the endpoint is auto-disabled (critical alert) and you re-enable it from the panel.

Order: delivery order is NOT guaranteed (retries + concurrency). Order it yourself by createdAt (ISO-8601 Z) + sequence (monotonic per connection; 0 when connection is null).