refactor(person): thin deletePerson to lean on V71 DB cascade (#684)

Drop the application-layer sender/receiver detach from deletePerson — the
V71 ON DELETE constraints now enforce it. Remove the now-unused
reassignSenderToNull repository method and rewrite the unit test to assert
only the existence check plus deleteById (verifyNoMoreInteractions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -27,6 +27,7 @@ import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.argThat;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
 import static org.mockito.Mockito.when;
 
 @ExtendWith(MockitoExtension.class)
@@ -147,9 +148,11 @@ class PersonServiceTest {
 
         personService.deletePerson(id);
 
-        verify(personRepository).reassignSenderToNull(id);
-        verify(personRepository).deleteReceiverReferences(id);
+        // Integrity is enforced by V71's ON DELETE constraints — the service only checks
+        // existence then deletes; it no longer detaches sender/receiver references itself.
+        verify(personRepository).findById(id);
         verify(personRepository).deleteById(id);
+        verifyNoMoreInteractions(personRepository);
     }
 
     @Test