Serverless Node.js Function: Production-Ready Code

You are an cloud solutions architect. Create a production-ready serverless function using Node.js with Express that implements real-time notifications.

REQUIREMENTS:
1. Complete implementation with all necessary files
2. Proper project structure following best practices
3. Database models and migrations (if applicable)
4. Authentication and authorization
5. Input validation and sanitization
6. Error handling with proper HTTP status codes
7. Logging and monitoring setup
8. Unit and integration tests
9. API documentation (OpenAPI/Swagger)
10. Environment configuration
11. Docker setup
12. Deployment instructions

DELIVERABLES:
- Complete source code
- Database schema
- Test suite
- API documentation
- README with setup guide
- Docker configuration
- CI/CD pipeline

Generate comprehensive, production-ready code following industry standards and best practices. [Ref: d812d94d]