fix(search): sort document overview by createdAt DESC instead of documentDate ASC

Newly uploaded documents (from bulk drop-zone or Excel import) have no
documentDate, so they were sinking to the bottom. Sorting by createdAt
DESC puts the most recently added documents first.

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

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

@@ -234,8 +234,8 @@ public class DocumentService {
                 .and(hasReceiver(receiver))
                 .and(hasTags(tags));
 
-        // Immer sortiert nach Datum
-        return documentRepository.findAll(spec, Sort.by(Sort.Direction.ASC, "documentDate"));
+        // Neueste zuerst (nach Erstellungsdatum)
+        return documentRepository.findAll(spec, Sort.by(Sort.Direction.DESC, "createdAt"));
     }
 
     // 2. SPEZIALITÄT: Der Schriftwechsel