feat(backend): replace fastapi backend with micronaut/java + mybatis

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

.gitignore

@@ -7,3 +7,18 @@ __pycache__/
 .trae/
 .vite/
 dist/
+
+# Java / Maven
+target/
+*.class
+*.jar
+*.war
+*.log
+.settings/
+.project
+.classpath
+*.iml
+.idea/
+*.gradle
+build/
+