test(stammbaum): integration tests for relationship constraints

@DataJpaTest + Postgres Testcontainer; 7 cases per Sara blocker 1:
- addRelationship_stores_and_is_readable
- addRelationship_throws_409_when_duplicate (unique_rel)
- addRelationship_throws_409_when_circular_parent
- deleteRelationship_throws_403_when_rel_belongs_to_different_person
- deleteRelationship_succeeds_for_symmetric_type_from_either_side
- setFamilyMember_true_makes_person_appear_in_network
- delete_person_cascades_to_relationships

Service now uses saveAndFlush so the unique_rel violation surfaces
synchronously inside the @Transactional method (otherwise the
DataIntegrityViolation fires at commit time, outside the try-catch).
Unit-test mocks updated accordingly.

Backend suite: 1406/1406 green.

Refs #358.

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

backend/src/main/java/org/raddatz/familienarchiv/relationship/RelationshipService.java

@@ -107,7 +107,9 @@ public class RelationshipService {
                 .build();
 
         try {
-            return toDTO(relationshipRepository.save(rel));
+            // saveAndFlush so the unique_rel constraint violates synchronously and is
+            // caught here, not at commit time outside the @Transactional boundary.
+            return toDTO(relationshipRepository.saveAndFlush(rel));
         } catch (DataIntegrityViolationException e) {
             throw DomainException.conflict(
                     ErrorCode.DUPLICATE_RELATIONSHIP,