refactor(transcription): drop dead existsById orphan guard from listener

Felix #2 / Markus #1 (PR #366 review). In the synchronous-transactional
path the existsById check could never return false — the rename and the
propagation share one transaction, so the renamed Person is guaranteed to
still exist when the listener runs. The check was forward-protection for
an eventual @Async refactor but its presence today is misleading: it
suggests a runtime branch that no test could reach against the real flow.

Delete the call, drop the PersonService dependency from the listener, drop
the now-unused PersonService.existsById, and remove the orphan-guard test
(it asserted a behaviour that the synchronous path cannot produce). When
async is added later the guard re-enters the codebase deliberately as part
of that refactor.

Refs #362 #366

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

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

@@ -33,16 +33,10 @@ import java.util.regex.Pattern;
 public class PersonMentionPropagationListener {
 
     private final TranscriptionBlockRepository blockRepository;
-    private final PersonService personService;
 
     @EventListener
     @Transactional
     public void onPersonDisplayNameChanged(PersonDisplayNameChangedEvent event) {
-        if (!personService.existsById(event.personId())) {
-            log.warn("Skipping mention propagation for non-existent personId {}", event.personId());
-            return;
-        }
-
         List<TranscriptionBlock> blocks =
                 blockRepository.findByMentionedPersons_PersonId(event.personId());
         if (blocks.isEmpty()) {

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

@@ -51,10 +51,6 @@ public class PersonService {
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.PERSON_NOT_FOUND, "Person not found: " + id));
     }
 
-    public boolean existsById(UUID id) {
-        return personRepository.existsById(id);
-    }
-
     public List<Person> findCorrespondents(UUID personId, String q) {
         if (q != null && !q.isBlank()) {
             return personRepository.findCorrespondentsWithFilter(personId, q);

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

@@ -26,9 +26,6 @@ import java.util.List;
 import java.util.UUID;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
 
 @DataJpaTest
 @AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@@ -42,16 +39,13 @@ class PersonMentionPropagationListenerTest {
     @Autowired EntityManager em;
 
     private PersonMentionPropagationListener listener;
-    private PersonService personService;
 
     private UUID documentId;
     private UUID annotationId;
 
     @BeforeEach
     void setUp() {
-        personService = mock(PersonService.class);
-        when(personService.existsById(any())).thenReturn(true);
-        listener = new PersonMentionPropagationListener(blockRepository, personService);
+        listener = new PersonMentionPropagationListener(blockRepository);
 
         Document doc = documentRepository.save(Document.builder()
                 .title("Letter").originalFilename("letter.pdf")
@@ -170,28 +164,6 @@ class PersonMentionPropagationListenerTest {
                 .containsExactly("Augusta Raddatz");
     }
 
-    @Test
-    void noOps_whenPersonIdNoLongerExists_orphanedSidecarGuard() {
-        UUID orphanId = UUID.randomUUID();
-        when(personService.existsById(orphanId)).thenReturn(false);
-
-        TranscriptionBlock saved = saveBlock(
-                "Stale reference to @Ghost Name should not be rewritten.",
-                List.of(new PersonMention(orphanId, "Ghost Name")));
-        em.clear();
-
-        listener.onPersonDisplayNameChanged(
-                new PersonDisplayNameChangedEvent(orphanId, "Ghost Name", "Resurrected Name"));
-        blockRepository.flush();
-        em.clear();
-
-        TranscriptionBlock reloaded = blockRepository.findById(saved.getId()).orElseThrow();
-        assertThat(reloaded.getText()).isEqualTo("Stale reference to @Ghost Name should not be rewritten.");
-        assertThat(reloaded.getMentionedPersons())
-                .extracting(PersonMention::getDisplayName)
-                .containsExactly("Ghost Name");
-    }
-
     @Test
     void propagatesAcross200Blocks_inUnderTwoSeconds_latencyFloor() {
         UUID personId = savedPersonId("Auguste", "Raddatz");