refactor(transcription): listener @Component → @Service

Markus #6 (PR #366 review). The class lives in service/ and is service-tier
business logic — wire-by-stereotype consistency calls for @Service. Both
annotations participate in @ComponentScan equivalently, so the bean
registration is unchanged.

Refs #362 #366

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

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

@@ -7,7 +7,7 @@ import org.raddatz.familienarchiv.model.PersonMention;
 import org.raddatz.familienarchiv.model.TranscriptionBlock;
 import org.raddatz.familienarchiv.repository.TranscriptionBlockRepository;
 import org.springframework.context.event.EventListener;
-import org.springframework.stereotype.Component;
+import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
 import java.util.List;
@@ -27,7 +27,7 @@ import java.util.regex.Pattern;
  * {@code @TransactionalEventListener(AFTER_COMMIT) + @Async} — one annotation
  * change.
  */
-@Component
+@Service
 @RequiredArgsConstructor
 @Slf4j
 public class PersonMentionPropagationListener {