feat(backend): dispatch thumbnail generation from MassImportService

ODS/Excel imports that actually upload a file (file.isPresent()) now
trigger thumbnail generation alongside hash/metadata. Metadata-only
import rows produce no thumbnail — nothing to render.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/service/MassImportService.java

@@ -59,6 +59,7 @@ public class MassImportService {
     private final PersonService personService;
     private final TagService tagService;
     private final S3Client s3Client;
+    private final ThumbnailAsyncRunner thumbnailAsyncRunner;
 
     @Value("${app.s3.bucket}")
     private String bucketName;
@@ -332,7 +333,10 @@ public class MassImportService {
         if (tag != null) doc.getTags().add(tag);
         doc.setMetadataComplete(metadataComplete);
 
-        documentRepository.save(doc);
+        Document saved = documentRepository.save(doc);
+        if (file.isPresent()) {
+            thumbnailAsyncRunner.dispatchAfterCommit(saved.getId());
+        }
         log.info("Importiert{}: {}", file.isEmpty() ? " (nur Metadaten)" : "", originalFilename);
     }