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

@@ -126,6 +126,7 @@ public class Document {
     private Set<TrainingLabel> trainingLabels = new HashSet<>();
 
     public String getThumbnailUrl() {
-        return null;
+        if (thumbnailKey == null) return null;
+        return "/api/documents/" + id + "/thumbnail";
     }
 }

backend/src/test/java/org/raddatz/familienarchiv/model/DocumentTest.java

@@ -20,4 +20,20 @@ class DocumentTest {
 
         assertThat(doc.getThumbnailUrl()).isNull();
     }
+
+    @Test
+    void getThumbnailUrl_omitsCacheBuster_whenThumbnailKeyPresentButGeneratedAtNull() {
+        UUID id = UUID.fromString("11111111-2222-3333-4444-555555555555");
+        Document doc = Document.builder()
+                .id(id)
+                .title("Brief")
+                .originalFilename("brief.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .thumbnailKey("thumbnails/" + id + ".jpg")
+                .thumbnailGeneratedAt(null)
+                .build();
+
+        assertThat(doc.getThumbnailUrl())
+                .isEqualTo("/api/documents/" + id + "/thumbnail");
+    }
 }