test(document): add query-count assertions for findAll + findById entity graphs

Adds Hibernate statistics to the test config and two new tests in
DocumentRepositoryTest:
- findAll_withSpecAndPageable asserts ≤5 statements for 10 documents
  (currently RED: EAGER @ManyToMany generates 31 secondary SELECTs)
- findById regression guard verifies collections load in ≤2 statements

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

backend/src/test/java/org/raddatz/familienarchiv/document/DocumentRepositoryTest.java

@@ -1,5 +1,9 @@
 package org.raddatz.familienarchiv.document;
 
+import jakarta.persistence.EntityManager;
+import jakarta.persistence.EntityManagerFactory;
+import org.hibernate.SessionFactory;
+import org.hibernate.stat.Statistics;
 import org.junit.jupiter.api.Test;
 import org.raddatz.familienarchiv.PostgresContainerConfig;
 import org.raddatz.familienarchiv.config.FlywayConfig;
@@ -21,6 +25,7 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.jdbc.test.autoconfigure.AutoConfigureTestDatabase;
 import org.springframework.boot.data.jpa.test.autoconfigure.DataJpaTest;
 import org.springframework.context.annotation.Import;
+import org.springframework.data.jpa.domain.Specification;
 
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageRequest;
@@ -55,6 +60,12 @@ class DocumentRepositoryTest {
     @Autowired
     private TranscriptionBlockRepository transcriptionBlockRepository;
 
+    @Autowired
+    private EntityManagerFactory entityManagerFactory;
+
+    @Autowired
+    private EntityManager entityManager;
+
     // ─── save and findById ────────────────────────────────────────────────────
 
     @Test
@@ -490,6 +501,65 @@ class DocumentRepositoryTest {
         assertThat(ids).containsExactlyInAnyOrder(grandparent.getId(), parent2.getId(), child2.getId());
     }
 
+    // ─── query-count — entity-graph assertions ────────────────────────────────
+
+    @Test
+    void findAll_withSpecAndPageable_loadsDocumentsInAtMostFiveStatements() {
+        Person sender = personRepository.save(Person.builder().firstName("Hans").lastName("QcSender").build());
+        Person receiver = personRepository.save(Person.builder().firstName("Anna").lastName("QcReceiver").build());
+        Tag tag = tagRepository.save(Tag.builder().name("QcTag").build());
+        for (int i = 0; i < 10; i++) {
+            documentRepository.save(Document.builder()
+                    .title("QcDoc " + i).originalFilename("qcdoc" + i + ".pdf")
+                    .status(DocumentStatus.UPLOADED)
+                    .sender(sender)
+                    .receivers(new HashSet<>(Set.of(receiver)))
+                    .tags(new HashSet<>(Set.of(tag)))
+                    .build());
+        }
+        entityManager.flush();
+        entityManager.clear();
+        Statistics stats = entityManagerFactory.unwrap(SessionFactory.class).getStatistics();
+        stats.setStatisticsEnabled(true);
+        stats.clear();
+
+        Specification<Document> allDocs = (root, query, cb) -> null;
+        documentRepository.findAll(allDocs, PageRequest.of(0, 10));
+
+        assertThat(stats.getPrepareStatementCount())
+                .as("@EntityGraph(Document.list) must load 10 docs in ≤5 statements, not N+1")
+                .isLessThanOrEqualTo(5);
+    }
+
+    @Test
+    void findById_loadsSenderReceiversAndTagsInAtMostTwoStatements() {
+        Person sender = personRepository.save(Person.builder().firstName("Max").lastName("FbSender").build());
+        Set<Person> receivers = new HashSet<>();
+        for (int i = 0; i < 3; i++) {
+            receivers.add(personRepository.save(
+                    Person.builder().firstName("R" + i).lastName("FbReceiver").build()));
+        }
+        Set<Tag> tags = new HashSet<>();
+        for (int i = 0; i < 5; i++) {
+            tags.add(tagRepository.save(Tag.builder().name("FbTag" + i).build()));
+        }
+        Document doc = documentRepository.save(Document.builder()
+                .title("FindByIdQc").originalFilename("findbyid_qc.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .sender(sender).receivers(receivers).tags(tags)
+                .build());
+        entityManager.flush();
+        entityManager.clear();
+        Statistics stats = entityManagerFactory.unwrap(SessionFactory.class).getStatistics();
+        stats.clear();
+
+        documentRepository.findById(doc.getId());
+
+        assertThat(stats.getPrepareStatementCount())
+                .as("@EntityGraph(Document.full) must load sender+receivers+tags in ≤2 statements, not 4")
+                .isLessThanOrEqualTo(2);
+    }
+
     // ─── seeding helpers ─────────────────────────────────────────────────────
 
     private Document uploaded(String title) {

backend/src/test/resources/application-test.yaml

@@ -13,6 +13,10 @@ spring:
     password: test
   mail:
     host: localhost
+  jpa:
+    properties:
+      hibernate:
+        generate_statistics: true
 
 # Disable OTel SDK entirely in tests — prevents auto-configuration from loading resource providers
 # (e.g. AzureAppServiceResourceProvider) that fail against the semconv version used here.