feat(document): add DocumentSort.UPDATED_AT for reader dashboard feed

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

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

@@ -727,6 +727,7 @@ public class DocumentService {
         return switch (sort) {
             case TITLE -> Sort.by(direction, "title");
             case UPLOAD_DATE -> Sort.by(direction, "createdAt");
+            case UPDATED_AT -> Sort.by(direction, "updatedAt");
             default -> Sort.by(direction, "documentDate");
         };
     }

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

@@ -1,5 +1,5 @@
 package org.raddatz.familienarchiv.document;
 
 public enum DocumentSort {
-    DATE, TITLE, SENDER, RECEIVER, UPLOAD_DATE, RELEVANCE
+    DATE, TITLE, SENDER, RECEIVER, UPLOAD_DATE, UPDATED_AT, RELEVANCE
 }

backend/src/main/resources/db/migration/V61__add_idx_documents_updated_at.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_documents_updated_at ON documents(updated_at DESC);