feat(person): add findByDisplayNameContaining service method

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

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

@@ -99,6 +99,10 @@ public class PersonService {
         return personRepository.findAllById(ids);
     }
 
+    public List<Person> findByDisplayNameContaining(String fragment) {
+        return personRepository.searchByName(fragment);
+    }
+
     public List<Person> findAllFamilyMembers() {
         return personRepository.findByFamilyMemberTrueOrderByLastNameAscFirstNameAsc();
     }