feat(dto): add uploadedAt to IncompleteDocumentDTO

Mapper populates uploadedAt from Document.createdAt so the dashboard
enrichment block can show a relative-time meta line ("vor 2 Min.")
per issue #296.

LocalDateTime matches the convention used by NotificationDTO,
DocumentVersionSummary and InviteListItemDTO.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/service/DocumentServiceTest.java

@@ -390,6 +390,22 @@ class DocumentServiceTest {
         assertThat(result.get(0).title()).isEqualTo("Unvollständig");
     }
 
+    @Test
+    void findIncompleteDocuments_mapsUploadedAtFromCreatedAt() {
+        java.time.LocalDateTime createdAt = java.time.LocalDateTime.of(2026, 4, 20, 12, 0);
+        Document doc = Document.builder()
+                .id(UUID.randomUUID())
+                .title("Recent")
+                .createdAt(createdAt)
+                .build();
+        when(documentRepository.findByMetadataCompleteFalse(any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of(doc)));
+
+        List<IncompleteDocumentDTO> result = documentService.findIncompleteDocuments(3);
+
+        assertThat(result.get(0).uploadedAt()).isEqualTo(createdAt);
+    }
+
     @Test
     void findIncompleteDocuments_passesSizeToPageable() {
         when(documentRepository.findByMetadataCompleteFalse(any(Pageable.class)))