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);

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

@@ -1403,6 +1403,21 @@ class DocumentServiceTest {
         assertThat(result.items()).hasSize(1); // only the slice is enriched
     }
 
+    @Test
+    void searchDocuments_UPDATED_AT_sort_resolves_to_updatedAt_field() {
+        ArgumentCaptor<Pageable> captor = ArgumentCaptor.forClass(Pageable.class);
+        when(documentRepository.findAll(any(org.springframework.data.jpa.domain.Specification.class), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of()));
+
+        documentService.searchDocuments(null, null, null, null, null, null, null, null,
+                DocumentSort.UPDATED_AT, "DESC", null,
+                org.springframework.data.domain.PageRequest.of(0, 5));
+
+        verify(documentRepository).findAll(any(org.springframework.data.jpa.domain.Specification.class), captor.capture());
+        assertThat(captor.getValue().getSort())
+                .isEqualTo(Sort.by(Sort.Direction.DESC, "updatedAt"));
+    }
+
     @Test
     void searchDocuments_senderSort_slicesInMemoryAndReportsFullTotal() {
         // Fixture: 120 docs with senders; request page 1, size 50 → expect 50 items