test(annotations): verify save() is called in updateAnnotation test [M5]

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

backend/src/test/java/org/raddatz/familienarchiv/service/AnnotationServiceTest.java

@@ -238,6 +238,7 @@ class AnnotationServiceTest {
         assertThat(result.getY()).isEqualTo(0.6);
         assertThat(result.getWidth()).isEqualTo(0.3);  // unchanged
         assertThat(result.getHeight()).isEqualTo(0.4); // unchanged
+        verify(annotationRepository).save(annotation);
     }
 
     // ─── listAnnotations ──────────────────────────────────────────────────────