feat(person): updatePerson publishes PersonDisplayNameChangedEvent on display-name change

PersonService now emits a domain event whenever Person.getDisplayName()
flips during an update. The snapshot is taken before the setter chain so we
compare like-for-like against the post-save value, and the event only
publishes when the two strings differ.

The test captures the published event via ArgumentCaptor and asserts the
title flip from "Herr" to "Frau" reaches the publisher with the correct
personId, oldDisplayName, and newDisplayName. Title participates in
DisplayNameFormatter, so this is the canonical case for "rename triggered
by something other than first/last name."

Implements PR-A tasks 9 and 10 as one red→green cycle (the test drove the
production change). Subsequent commits cover the negative cases (alias /
notes only) and the propagation listener that consumes the event.

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/service/PersonService.java

@@ -13,11 +13,13 @@ import org.raddatz.familienarchiv.dto.PersonUpdateDTO;
 import org.raddatz.familienarchiv.exception.DomainException;
 import org.raddatz.familienarchiv.exception.ErrorCode;
 import org.raddatz.familienarchiv.model.Person;
+import org.raddatz.familienarchiv.model.PersonDisplayNameChangedEvent;
 import org.raddatz.familienarchiv.model.PersonNameAlias;
 import org.raddatz.familienarchiv.model.PersonNameAliasType;
 import org.raddatz.familienarchiv.model.PersonType;
 import org.raddatz.familienarchiv.repository.PersonNameAliasRepository;
 import org.raddatz.familienarchiv.repository.PersonRepository;
+import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
@@ -31,6 +33,7 @@ public class PersonService {
 
     private final PersonRepository personRepository;
     private final PersonNameAliasRepository aliasRepository;
+    private final ApplicationEventPublisher eventPublisher;
 
     public List<PersonSummaryDTO> findAll(String q) {
         if (q == null) {
@@ -157,6 +160,7 @@ public class PersonService {
         validateYears(dto.getBirthYear(), dto.getDeathYear());
         Person person = personRepository.findById(id)
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.PERSON_NOT_FOUND, "Person not found: " + id));
+        String oldDisplayName = person.getDisplayName();
         person.setPersonType(dto.getPersonType());
         person.setTitle(dto.getTitle() == null || dto.getTitle().isBlank() ? null : dto.getTitle().trim());
         person.setFirstName(dto.getFirstName());
@@ -165,7 +169,12 @@ public class PersonService {
         person.setNotes(dto.getNotes() == null || dto.getNotes().isBlank() ? null : dto.getNotes().trim());
         person.setBirthYear(dto.getBirthYear());
         person.setDeathYear(dto.getDeathYear());
-        return personRepository.save(person);
+        Person saved = personRepository.save(person);
+        String newDisplayName = saved.getDisplayName();
+        if (!Objects.equals(oldDisplayName, newDisplayName)) {
+            eventPublisher.publishEvent(new PersonDisplayNameChangedEvent(id, oldDisplayName, newDisplayName));
+        }
+        return saved;
     }
 
     @Transactional

backend/src/test/java/org/raddatz/familienarchiv/service/PersonServiceTest.java

@@ -2,6 +2,7 @@ package org.raddatz.familienarchiv.service;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
 import org.mockito.InjectMocks;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
@@ -10,11 +11,13 @@ import org.raddatz.familienarchiv.dto.PersonSummaryDTO;
 import org.raddatz.familienarchiv.dto.PersonUpdateDTO;
 import org.raddatz.familienarchiv.exception.DomainException;
 import org.raddatz.familienarchiv.model.Person;
+import org.raddatz.familienarchiv.model.PersonDisplayNameChangedEvent;
 import org.raddatz.familienarchiv.model.PersonNameAlias;
 import org.raddatz.familienarchiv.model.PersonNameAliasType;
 import org.raddatz.familienarchiv.model.PersonType;
 import org.raddatz.familienarchiv.repository.PersonNameAliasRepository;
 import org.raddatz.familienarchiv.repository.PersonRepository;
+import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.web.server.ResponseStatusException;
 
 import java.util.List;
@@ -31,6 +34,7 @@ class PersonServiceTest {
 
     @Mock PersonRepository personRepository;
     @Mock PersonNameAliasRepository aliasRepository;
+    @Mock ApplicationEventPublisher eventPublisher;
     @InjectMocks PersonService personService;
 
     // ─── getById ─────────────────────────────────────────────────────────────
@@ -242,6 +246,37 @@ class PersonServiceTest {
         assertThat(result.getAlias()).isEqualTo("Anna Alt");
     }
 
+    // ─── updatePerson (display-name change event) ────────────────────────────
+
+    @Test
+    void updatePerson_publishesEvent_whenTitleChanges() {
+        UUID id = UUID.randomUUID();
+        Person existing = Person.builder()
+                .id(id).title("Herr").firstName("Auguste").lastName("Raddatz")
+                .personType(PersonType.PERSON).build();
+        String oldName = existing.getDisplayName();
+
+        when(personRepository.findById(id)).thenReturn(Optional.of(existing));
+        when(personRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        PersonUpdateDTO dto = new PersonUpdateDTO();
+        dto.setPersonType(PersonType.PERSON);
+        dto.setTitle("Frau"); dto.setFirstName("Auguste"); dto.setLastName("Raddatz");
+
+        personService.updatePerson(id, dto);
+
+        ArgumentCaptor<PersonDisplayNameChangedEvent> captor =
+                ArgumentCaptor.forClass(PersonDisplayNameChangedEvent.class);
+        verify(eventPublisher).publishEvent(captor.capture());
+
+        PersonDisplayNameChangedEvent event = captor.getValue();
+        assertThat(event.personId()).isEqualTo(id);
+        assertThat(event.oldDisplayName()).isEqualTo(oldName);
+        assertThat(event.newDisplayName())
+                .isNotEqualTo(oldName)
+                .contains("Frau");
+    }
+
     // ─── findOrCreateByAlias ─────────────────────────────────────────────────
 
     @Test