fix(test): update optimistic-lock mock to use JOIN FETCH query method

PersonServiceTest wired the mock on findByMentionedPersons_PersonId; the listener
now calls findByPersonIdWithMentionsFetched so the mock returned an empty list,
suppressing the saveAllAndFlush call and breaking the exception-propagation test.

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

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

@@ -321,7 +321,7 @@ class PersonServiceTest {
                 .build();
 
         TranscriptionBlockRepository blockRepo = mock(TranscriptionBlockRepository.class);
-        when(blockRepo.findByMentionedPersons_PersonId(id))
+        when(blockRepo.findByPersonIdWithMentionsFetched(id))
                 .thenReturn(List.of(referencingBlock));
         when(blockRepo.saveAllAndFlush(any()))
                 .thenThrow(new ObjectOptimisticLockingFailureException(