test(document): assert non-empty result in receiverSort lazy-loading test

assertThatCode(() -> service.searchDocuments(...)) passed vacuously on an
empty page; capture the result, assert totalElements > 0, then assert
getSender().getLastName() is accessible post-return.

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

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

@@ -119,10 +119,13 @@ class DocumentLazyLoadingTest {
                 .tags(new HashSet<>(Set.of(tag)))
                 .build());
 
-        assertThatCode(() -> documentService.searchDocuments(
+        DocumentSearchResult result = documentService.searchDocuments(
                 null, null, null, null, null, null, null, null,
                 DocumentSort.RECEIVER, "asc", null,
-                PageRequest.of(0, 20)))
+                PageRequest.of(0, 20));
+        assertThat(result.totalElements()).isGreaterThan(0);
+        assertThatCode(() ->
+                result.items().forEach(i -> i.document().getSender().getLastName()))
                 .doesNotThrowAnyException();
     }