Integrate RabbitMQ for background jobs (notifications, payments)

Integrate RabbitMQ for asynchronous background job processing.

**Requirements:**
- Setup RabbitMQ message broker
- Create message queues for notifications, payments, and emails
- Implement job producers (API endpoints that trigger jobs)
- Implement job consumers/workers
- Add retry logic and dead letter queue handling
- Setup monitoring for queue health

**Deliverables:**
- RabbitMQ configuration
- Message producer service
- Worker/consumer implementations
- Job definitions for notifications, payments, and emails
- Error handling and retry policies