Implement Webhook System

Implement an outgoing webhook system for {{PROJECT_CONTEXT}}:

Features:
1. Webhook registration (URL, events, secret)
2. Event triggering on specific actions
3. Payload signing for verification
4. Retry logic with exponential backoff
5. Delivery log with status tracking
6. Webhook management UI (create, edit, delete, test)
7. Rate limiting per webhook

Provide:
- Database schema for webhooks and delivery logs
- Webhook dispatcher service
- Payload signing (HMAC-SHA256)
- Retry queue with backoff
- Management API endpoints
- Admin UI for webhook management
- Test webhook endpoint
- Delivery status dashboard