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);