Update API Gateway routing and testing

Description:
Integrate AI assistant with API Gateway and create comprehensive tests.

**Acceptance Criteria:**
- [ ] Add routing rules to API Gateway for /api/ai/**
- [ ] Create REST controller with /api/ai/chat endpoint
- [ ] Implement request/response DTOs
- [ ] Add integration tests
- [ ] Create Postman collection for testing
- [ ] Update documentation with usage examples 