Webhook Delivery System

Build a webhook delivery system for:

**Events:** [EVENT_TYPES — e.g., "order.created, payment.completed, user.signup"]
**Expected subscribers:** [SUBSCRIBER_COUNT]

Generate:
1. **Webhook registration** — API for subscribers to register endpoints and select events
2. **Payload format** — standardized event envelope with signature verification
3. **Delivery engine** — async delivery with configurable timeout
4. **Retry policy** — exponential backoff (1m, 5m, 30m, 2h, 24h) with dead letter
5. **Signature verification** — HMAC-SHA256 signing so receivers can verify authenticity
6. **Delivery log** — track every attempt with status code, response time, and body
7. **Circuit breaker** — disable endpoints that fail consistently
8. **Testing endpoint** — let subscribers send test events to verify their handler

Include TypeScript implementation for the webhook manager and delivery worker.