test(document): getThumbnailUrl returns null when thumbnailKey is null

First TDD step for centralising the thumbnail URL convention on the
Document entity (#309). Adds a stub getter returning null and a test
that locks the "no key → no URL" branch.

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

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

@@ -131,4 +131,8 @@ public class Document {
     @Enumerated(EnumType.STRING)
     @Builder.Default
     private Set<TrainingLabel> trainingLabels = new HashSet<>();
+
+    public String getThumbnailUrl() {
+        return null;
+    }
 }