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

backend/index.css

@@ -0,0 +1,16 @@
+html {
+    box-sizing: border-box;
+    overflow: -moz-scrollbars-vertical;
+    overflow-y: scroll;
+}
+
+*,
+*:before,
+*:after {
+    box-sizing: inherit;
+}
+
+body {
+    margin: 0;
+    background: #fafafa;
+}