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

@@ -124,4 +124,8 @@ public class Document {
     @Enumerated(EnumType.STRING)
     @Builder.Default
     private Set<TrainingLabel> trainingLabels = new HashSet<>();
+
+    public String getThumbnailUrl() {
+        return null;
+    }
 }

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

@@ -0,0 +1,23 @@
+package org.raddatz.familienarchiv.model;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DocumentTest {
+
+    @Test
+    void getThumbnailUrl_returnsNull_whenThumbnailKeyNull() {
+        Document doc = Document.builder()
+                .id(UUID.randomUUID())
+                .title("Brief")
+                .originalFilename("brief.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .thumbnailKey(null)
+                .build();
+
+        assertThat(doc.getThumbnailUrl()).isNull();
+    }
+}