refactor(document): address review concerns from PR #660

- Restore JavaDoc on DocumentSearchResult.of() and .paged() factory methods
- Remove redundant null guards on @Builder.Default collections in toListItem()
- Map DocumentListItem fields explicitly in DocumentMultiSelect before cast
- Add DocumentListItem required fields to docFactory in spec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentSearchResult.java

@@ -17,11 +17,19 @@ public record DocumentSearchResult(
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
         int totalPages
 ) {
+    /**
+     * Single-page convenience factory used by empty-result shortcuts and by tests that
+     * don't care about paging. Treats the whole list as page 0 of itself.
+     */
     public static DocumentSearchResult of(List<DocumentListItem> items) {
         int size = items.size();
         return new DocumentSearchResult(items, size, 0, size, size == 0 ? 0 : 1);
     }
 
+    /**
+     * Paged factory used by the service when it has a real Pageable + full match count
+     * (e.g. from Spring's Page&lt;T&gt; or from an in-memory sort-then-slice).
+     */
     public static DocumentSearchResult paged(List<DocumentListItem> slice, Pageable pageable, long totalElements) {
         int pageSize = pageable.getPageSize();
         int totalPages = pageSize == 0 ? 0 : (int) ((totalElements + pageSize - 1) / pageSize);

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

@@ -759,8 +759,8 @@ public class DocumentService {
                 doc.getThumbnailUrl(),
                 doc.getDocumentDate(),
                 doc.getSender(),
-                doc.getReceivers() != null ? List.copyOf(doc.getReceivers()) : List.of(),
-                doc.getTags() != null ? List.copyOf(doc.getTags()) : List.of(),
+                List.copyOf(doc.getReceivers()),
+                List.copyOf(doc.getTags()),
                 doc.getArchiveBox(),
                 doc.getArchiveFolder(),
                 doc.getLocation(),