refactor(document): remove dead DocumentService.updateThumbnailMetadata()

No production code calls this method since ThumbnailService was changed
to write thumbnail metadata via documentRepository.save() directly.
Removing the unreachable wrapper eliminates false coverage and noise
during future security audits.

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

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java

@@ -92,10 +92,6 @@ public class DocumentService {
         return documentRepository.findByFilePathIsNotNullAndThumbnailKeyIsNull();
     }
 
-    public Document updateThumbnailMetadata(Document doc) {
-        return documentRepository.save(doc);
-    }
-
     public Optional<Document> findByOriginalFilename(String originalFilename) {
         return documentRepository.findByOriginalFilename(originalFilename);
     }