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/swagger-initializer.js

@@ -0,0 +1,20 @@
+window.onload = function() {
+  //<editor-fold desc="Changeable Configuration Block">
+
+  // the following lines will be replaced by docker/configurator, when it runs in a docker-container
+  window.ui = SwaggerUIBundle({
+    url: "https://petstore.swagger.io/v2/swagger.json",
+    dom_id: '#swagger-ui',
+    deepLinking: true,
+    presets: [
+      SwaggerUIBundle.presets.apis,
+      SwaggerUIStandalonePreset
+    ],
+    plugins: [
+      SwaggerUIBundle.plugins.DownloadUrl
+    ],
+    layout: "StandaloneLayout"
+  });
+
+  //</editor-fold>
+};