Refactor domain models to records and move exceptions to sub-package

- Convert Event and Rsvp from mutable POJOs to Java records
- Move all 8 exception classes to application.service.exception sub-package
- Add ArchUnit rule enforcing domain models must be records

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/test/java/de/fete/HexagonalArchitectureTest.java

@@ -4,10 +4,14 @@ import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes;
 import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.noClasses;
 import static com.tngtech.archunit.library.Architectures.onionArchitecture;
 
+import com.tngtech.archunit.core.domain.JavaClass;
 import com.tngtech.archunit.core.importer.ImportOption;
 import com.tngtech.archunit.junit.AnalyzeClasses;
 import com.tngtech.archunit.junit.ArchTest;
+import com.tngtech.archunit.lang.ArchCondition;
 import com.tngtech.archunit.lang.ArchRule;
+import com.tngtech.archunit.lang.ConditionEvents;
+import com.tngtech.archunit.lang.SimpleConditionEvent;
 
 @AnalyzeClasses(packages = "de.fete", importOptions = ImportOption.DoNotIncludeTests.class)
 class HexagonalArchitectureTest {
@@ -65,4 +69,24 @@ class HexagonalArchitectureTest {
   static final ArchRule webAdapterMustNotDependOnOutboundPorts = noClasses()
       .that().resideInAPackage("de.fete.adapter.in.web..")
       .should().dependOnClassesThat().resideInAPackage("de.fete.domain.port.out..");
+
+  @ArchTest
+  static final ArchRule domainModelsMustBeRecords = classes()
+      .that().resideInAPackage("de.fete.domain.model..")
+      .and().doNotHaveSimpleName("package-info")
+      .should(beRecords());
+
+  private static ArchCondition<JavaClass> beRecords() {
+    return new ArchCondition<>("be records") {
+      @Override
+      public void check(JavaClass javaClass,
+          ConditionEvents events) {
+        boolean isRecord = javaClass.reflect().isRecord();
+        if (!isRecord) {
+          events.add(SimpleConditionEvent.violated(javaClass,
+              javaClass.getFullName() + " is not a record"));
+        }
+      }
+    };
+  }
 }