test(document): enable statistics before findById query-count assertion

Without setStatisticsEnabled(true) the counter stays 0 and ≤2 passes
vacuously when the test runs in isolation.

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

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

@@ -551,6 +551,7 @@ class DocumentRepositoryTest {
         entityManager.flush();
         entityManager.clear();
         Statistics stats = entityManagerFactory.unwrap(SessionFactory.class).getStatistics();
+        stats.setStatisticsEnabled(true);
         stats.clear();
 
         documentRepository.findById(doc.getId());