docs(geschichte): clarify readOnly=true tx purpose on getById()

Add inline comment explaining that readOnly=true lets lazy collections
resolve within the same transaction when getById() is called from getView().

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

backend/src/main/java/org/raddatz/familienarchiv/geschichte/GeschichteService.java

@@ -68,6 +68,7 @@ public class GeschichteService {
         return geschichteRepository.count(GeschichteSpecifications.hasStatus(GeschichteStatus.PUBLISHED));
     }
 
+    // readOnly = true: lazy collections resolve within the same tx when called from getView()
     @Transactional(readOnly = true)
     public Geschichte getById(UUID id) {
         Geschichte g = geschichteRepository.findById(id)