fix(#148): flush entity manager after @Modifying queries in PersonRepositoryTest

Native queries bypass the JPA first-level cache; flush+clear is required before
reloading entities to see the updated state in the same transaction.

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

backend/src/test/java/org/raddatz/familienarchiv/repository/PersonRepositoryTest.java

@@ -11,6 +11,8 @@ import org.springframework.boot.jdbc.test.autoconfigure.AutoConfigureTestDatabas
 import org.springframework.boot.data.jpa.test.autoconfigure.DataJpaTest;
 import org.springframework.context.annotation.Import;
 
+import jakarta.persistence.EntityManager;
+import jakarta.persistence.PersistenceContext;
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
@@ -28,6 +30,9 @@ class PersonRepositoryTest {
     @Autowired
     private DocumentRepository documentRepository;
 
+    @PersistenceContext
+    private EntityManager entityManager;
+
     // ─── save and findById ────────────────────────────────────────────────────
 
     @Test
@@ -230,6 +235,8 @@ class PersonRepositoryTest {
                 .sender(source).build());
 
         personRepository.reassignSender(source.getId(), target.getId());
+        entityManager.flush();
+        entityManager.clear();
 
         List<Document> docs = documentRepository.findBySenderId(target.getId());
         assertThat(docs).hasSize(1);
@@ -250,6 +257,8 @@ class PersonRepositoryTest {
                 .sender(sender).receivers(Set.of(source)).build());
 
         personRepository.insertMissingReceiverReference(source.getId(), target.getId());
+        entityManager.flush();
+        entityManager.clear();
 
         Document reloaded = documentRepository.findById(doc.getId()).orElseThrow();
         assertThat(reloaded.getReceivers())
@@ -270,6 +279,8 @@ class PersonRepositoryTest {
                 .sender(sender).receivers(Set.of(source, target)).build());
 
         personRepository.insertMissingReceiverReference(source.getId(), target.getId());
+        entityManager.flush();
+        entityManager.clear();
 
         Document reloaded = documentRepository.findById(doc.getId()).orElseThrow();
         long targetCount = reloaded.getReceivers().stream()
@@ -294,6 +305,8 @@ class PersonRepositoryTest {
                 .sender(sender).receivers(Set.of(toDelete)).build());
 
         personRepository.deleteReceiverReferences(toDelete.getId());
+        entityManager.flush();
+        entityManager.clear();
 
         assertThat(documentRepository.findById(doc1.getId()).orElseThrow().getReceivers()).isEmpty();
         assertThat(documentRepository.findById(doc2.getId()).orElseThrow().getReceivers()).isEmpty();