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/main/java/org/raddatz/familienarchiv/dto/IncompleteDocumentDTO.java

@@ -2,9 +2,11 @@ package org.raddatz.familienarchiv.dto;
 
 import io.swagger.v3.oas.annotations.media.Schema;
 
+import java.time.LocalDateTime;
 import java.util.UUID;
 
 public record IncompleteDocumentDTO(
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) UUID id,
-        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String title
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String title,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) LocalDateTime uploadedAt
 ) {}

backend/src/main/java/org/raddatz/familienarchiv/service/DocumentService.java

@@ -542,7 +542,7 @@ public class DocumentService {
         PageRequest pageable = PageRequest.of(0, size, Sort.by(Sort.Direction.DESC, "createdAt"));
         return documentRepository.findByMetadataCompleteFalse(pageable)
                 .stream()
-                .map(doc -> new IncompleteDocumentDTO(doc.getId(), doc.getTitle()))
+                .map(doc -> new IncompleteDocumentDTO(doc.getId(), doc.getTitle(), doc.getCreatedAt()))
                 .toList();
     }