refactor(person): simplify mergePersons to lean on V71 cascade (#684)

Drop the explicit deleteReceiverReferences call from mergePersons — the
source's leftover receiver join rows now cascade-drop via V71's ON DELETE
CASCADE on deleteById. Remove the now-unused deleteReceiverReferences
repository method (and its repo test), and add clearAutomatically +
flushAutomatically to the remaining merge native queries so the L1 cache
cannot desync from the bulk updates. Rewrite the merge unit test with
verifyNoMoreInteractions and add an end-to-end merge regression test (AC-7).

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

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

@@ -189,12 +189,15 @@ public interface PersonRepository extends JpaRepository<Person, UUID> {
     List<Person> findCorrespondentsWithFilter(@Param("personId") UUID personId, @Param("q") String q);
 
     // --- Merge helpers (native SQL to bypass JPA entity layer) ---
+    // clearAutomatically + flushAutomatically keep the L1 cache from desyncing: these bulk
+    // updates run beneath Hibernate, and mergePersons follows them with a deleteById whose
+    // ON DELETE CASCADE (V71) also fires beneath the session.
 
-    @Modifying
+    @Modifying(clearAutomatically = true, flushAutomatically = true)
     @Query(value = "UPDATE documents SET sender_id = :target WHERE sender_id = :source", nativeQuery = true)
     void reassignSender(@Param("source") UUID source, @Param("target") UUID target);
 
-    @Modifying
+    @Modifying(clearAutomatically = true, flushAutomatically = true)
     @Query(value = """
             INSERT INTO document_receivers (document_id, person_id)
             SELECT document_id, :target FROM document_receivers
@@ -204,8 +207,4 @@ public interface PersonRepository extends JpaRepository<Person, UUID> {
               )
             """, nativeQuery = true)
     void insertMissingReceiverReference(@Param("source") UUID source, @Param("target") UUID target);
-
-    @Modifying
-    @Query(value = "DELETE FROM document_receivers WHERE person_id = :source", nativeQuery = true)
-    void deleteReceiverReferences(@Param("source") UUID source);
 }

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

@@ -293,6 +293,12 @@ public class PersonService {
         return personRepository.save(person);
     }
 
+    /**
+     * Merges the source person into the target, then deletes the source. Sender references move
+     * to the target; receiver references the target lacks are inserted. The source's leftover
+     * receiver join rows are not deleted explicitly — they cascade-drop via V71's
+     * {@code ON DELETE CASCADE} on {@code document_receivers.person_id} when the source is deleted.
+     */
     @Transactional
     public void mergePersons(UUID sourceId, UUID targetId) {
         if (sourceId.equals(targetId)) {
@@ -309,9 +315,7 @@ public class PersonService {
         // Add target as receiver where source is receiver but target is not yet
         personRepository.insertMissingReceiverReference(sourceId, targetId);
 
-        // Remove all remaining source receiver references (duplicates already handled)
-        personRepository.deleteReceiverReferences(sourceId);
-
+        // Source's remaining receiver rows cascade-drop via V71's ON DELETE CASCADE.
         personRepository.deleteById(sourceId);
     }
 

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

@@ -367,30 +367,6 @@ class PersonRepositoryTest {
         assertThat(result).hasSize(1);
     }
 
-    // ─── deleteReceiverReferences ─────────────────────────────────────────────
-
-    @Test
-    void deleteReceiverReferences_removesPersonFromAllDocumentReceivers() {
-        Person toDelete = personRepository.save(Person.builder().firstName("Weg").lastName("Person").build());
-        Person sender   = personRepository.save(Person.builder().firstName("Send").lastName("Er").build());
-
-        Document doc1 = documentRepository.save(Document.builder()
-                .title("Brief 1").originalFilename("b1.pdf")
-                .status(DocumentStatus.UPLOADED)
-                .sender(sender).receivers(Set.of(toDelete)).build());
-        Document doc2 = documentRepository.save(Document.builder()
-                .title("Brief 2").originalFilename("b2.pdf")
-                .status(DocumentStatus.UPLOADED)
-                .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();
-    }
-
     // ─── searchByName with aliases ───────────────────────────────────────────
 
     @Test

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

@@ -220,4 +220,38 @@ class PersonServiceIntegrationTest {
         // The other person and the documents themselves survive the delete.
         assertThat(personRepository.findById(bystander.getId())).isPresent();
     }
+
+    @Test
+    void mergePersons_targetInheritsReferences_sourceJoinRowCascadeDrops_noFkError() {
+        // AC-7: merging a source who is sender of A and receiver of B into a target leaves the
+        // target as sender of A and receiver of B, drops the source's leftover receiver row via
+        // V71's ON DELETE CASCADE (no explicit delete, no FK error), and co-receivers are intact.
+        Person source = personRepository.save(Person.builder().firstName("Anna").lastName("Alt").build());
+        Person target = personRepository.save(Person.builder().firstName("Anna").lastName("Neu").build());
+        Person coReceiver = personRepository.save(Person.builder().firstName("Mit").lastName("Empfänger").build());
+        Person sender = personRepository.save(Person.builder().firstName("Send").lastName("Er").build());
+
+        Document docA = documentRepository.save(Document.builder()
+                .title("Von Anna").originalFilename("a.pdf")
+                .status(DocumentStatus.UPLOADED).sender(source).build());
+        Document docB = documentRepository.save(Document.builder()
+                .title("An Anna").originalFilename("b.pdf")
+                .status(DocumentStatus.UPLOADED).sender(sender)
+                .receivers(new java.util.HashSet<>(Set.of(source, coReceiver))).build());
+        entityManager.flush();
+        entityManager.clear();
+
+        personService.mergePersons(source.getId(), target.getId());
+        entityManager.flush();
+        entityManager.clear();
+
+        assertThat(personRepository.findById(source.getId())).isEmpty();
+
+        Document reloadedA = documentRepository.findById(docA.getId()).orElseThrow();
+        assertThat(reloadedA.getSender().getId()).isEqualTo(target.getId());
+
+        Document reloadedB = documentRepository.findById(docB.getId()).orElseThrow();
+        assertThat(reloadedB.getReceivers()).extracting(Person::getId)
+                .containsExactlyInAnyOrder(target.getId(), coReceiver.getId());
+    }
 }

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

@@ -703,10 +703,14 @@ class PersonServiceTest {
 
         personService.mergePersons(sourceId, targetId);
 
+        verify(personRepository).findById(sourceId);
+        verify(personRepository).findById(targetId);
         verify(personRepository).reassignSender(sourceId, targetId);
         verify(personRepository).insertMissingReceiverReference(sourceId, targetId);
-        verify(personRepository).deleteReceiverReferences(sourceId);
         verify(personRepository).deleteById(sourceId);
+        // The source's leftover receiver rows cascade-drop via V71's ON DELETE CASCADE on
+        // deleteById — merge no longer deletes them explicitly.
+        verifyNoMoreInteractions(personRepository);
     }
 
     // ─── getAliases ─────────────────────────────────────────────────────────