This commit fully replaces the legacy Python FastAPI backend with a new Java-based Micronaut + MyBatis CRUD backend:
- add Maven build configuration and project dependencies
- implement database config, auto data initialization, and custom error handling
- add CRUD controllers, repository, mapper, and entity classes
- include Swagger UI static resources and configuration
- update README.md to reflect new backend setup and execution steps
- remove old Python backend files (main.py, requirements.txt, init_db.py)
- add Java/Maven-specific gitignore rules
