Design outbound webhook delivery

Help me design outbound webhook delivery.
Inputs: product; current state; requirements.

1. Define event envelopes and signature checks.
2. design idempotency, backoff and failure isolation.
3. provide replay, audit and delivery-status observability.

Deliver: Event model, Delivery pipeline, Security, Retries and endpoint health, Ordering and duplicates, Customer tooling, Open questions.
Clarify critical gaps, separate facts from assumptions, and do not invent data, citations or completed actions.

My material: [fill in here]

Before you start

Provide product; current state; requirements.