test(document): remove dead updateThumbnailMetadata test

ThumbnailService now calls documentRepository.save() directly.
DocumentService.updateThumbnailMetadata() has no production callers,
so its test describes behaviour that no longer exists in the
production path.

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

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

@@ -2301,17 +2301,6 @@ class DocumentServiceTest {
         assertThat(documentService.findForThumbnailBackfill()).containsExactly(a, b);
     }
 
-    // ─── updateThumbnailMetadata ───────────────────────────────────────────────
-
-    @Test
-    void updateThumbnailMetadata_savesDocument() {
-        Document doc = Document.builder().id(UUID.randomUUID()).title("T").build();
-        when(documentRepository.save(doc)).thenReturn(doc);
-
-        assertThat(documentService.updateThumbnailMetadata(doc)).isEqualTo(doc);
-        verify(documentRepository).save(doc);
-    }
-
     // ─── findByOriginalFilename ────────────────────────────────────────────────
 
     @Test