test(document): address @saraholt — lock JSON wire contract for thumbnailUrl

Prior coverage only exercised getThumbnailUrl() as a Java method call.
The new case serialises via ObjectMapper and asserts the resulting JSON
contains "thumbnailUrl":"..." so we catch silent breakages in the wire
contract (getter rename, @JsonIgnore, visibility drop) — not just
regressions in the method's return value (#309).

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

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

@@ -1,5 +1,7 @@
 package org.raddatz.familienarchiv.model;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 import org.junit.jupiter.api.Test;
 
 import java.time.LocalDateTime;
@@ -58,4 +60,26 @@ class DocumentTest {
         String expected = "/api/documents/" + id + "/thumbnail?v=2026-04-23T14%3A30%3A45";
         assertThat(doc.getThumbnailUrl()).isEqualTo(expected);
     }
+
+    @Test
+    void thumbnailUrl_isSerialisedToJson_soFrontendReceivesIt() throws Exception {
+        UUID id = UUID.fromString("99999999-aaaa-bbbb-cccc-111122223333");
+        Document doc = Document.builder()
+                .id(id)
+                .title("Brief")
+                .originalFilename("brief.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .thumbnailKey("thumbnails/" + id + ".jpg")
+                .thumbnailGeneratedAt(LocalDateTime.of(2026, 4, 23, 9, 0, 0))
+                .build();
+
+        ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());
+        String json = mapper.writeValueAsString(doc);
+
+        // Locks the wire contract, not just the Java API: every Document JSON must carry
+        // `thumbnailUrl`. Protects against silent breakage if the getter gets renamed,
+        // hidden behind @JsonIgnore, or visibility-reduced — any of which would leave the
+        // frontend rendering the fallback icon on every surface.
+        assertThat(json).contains("\"thumbnailUrl\":\"" + doc.getThumbnailUrl() + "\"");
+    }
 }