Add Webhook notification service

[big14way]

Create a webhook service that sends HTTP notifications when contract events occur.

Acceptance Criteria:

• Allow registering webhook URLs for specific event types
• POST event data to registered URLs when events are detected
• Include retry logic for failed deliveries (3 retries)
• Add webhook management API (register, list, delete)
• Support HMAC signature verification

Files: new directory webhook/, package.json

[big14way]

@robinshen36 youve been assigned

[lustsazeus-lab]

Nice acceptance criteria. One thing that pays off early is idempotency keys on deliveries + persisted delivery state (pending/sent/failed) before retries.

That avoids duplicate side effects when receivers timeout but still process the payload.