test(person): address PR #736 review nits

- AC-3 cascade test: assert an innocent bystander's mention row survives the
  delete, proving the cascade is scoped to the deleted person (Nora).
- Fix integration-test comment: receivers is @ManyToMany(LAZY), not an EAGER
  @ElementCollection (Sara).
- ADR-032: note the @ prefix is kept in the degraded path, stripped in live
  mentions (Leonie).
- Add trailing newline to PersonRepository.java (Felix).

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

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

@@ -207,4 +207,4 @@ public interface PersonRepository extends JpaRepository<Person, UUID> {
               )
             """, nativeQuery = true)
     void insertMissingReceiverReference(@Param("source") UUID source, @Param("target") UUID target);
-}
+}