Webhook Handler with Security

Webhook endpoint for [SERVICE A → SERVICE B]:

**Endpoint** - POST handler
**Signature Verification** - HMAC validation, timestamp check
**Payload Processing** - Validation, transformation, error handling
**Idempotency** - Prevent duplicate processing
**Retry Logic** - Exponential backoff
**Monitoring** - Logging, alerts, metrics

Include: Full code, tests, deployment.