feat(document): getThumbnailUrl composes /api/documents/{id}/thumbnail when key present

The no-cache-buster branch covers documents whose thumbnail key is set
but whose thumbnailGeneratedAt is still null — which only happens in
the narrow window between the key being persisted and the async worker
stamping the timestamp (#309).

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

backend/src/main/java/org/raddatz/familienarchiv/model/Document.java

@@ -133,6 +133,7 @@ public class Document {
     private Set<TrainingLabel> trainingLabels = new HashSet<>();
 
     public String getThumbnailUrl() {
-        return null;
+        if (thumbnailKey == null) return null;
+        return "/api/documents/" + id + "/thumbnail";
     }
 }