test(fts): add overflow guard and UUID-as-String regression tests

- searchDocuments_relevance_returns_empty_when_offset_exceeds_maxInt:
  proves the long→int guard fires and findFtsPageRaw is never called
- searchDocuments_relevance_handles_string_uuid_from_jdbc_driver:
  exercises the toFtsPage String fallback branch for JDBC drivers that
  return UUID columns as String instead of java.util.UUID

Addresses Sara's review concerns on PR #488.

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

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

@@ -124,6 +124,41 @@ class DocumentServiceSortTest {
         assertThat(result.items().get(0).document().getId()).isEqualTo(id1);
     }
 
+    // ─── RELEVANCE sort — overflow guard ─────────────────────────────────────
+
+    @Test
+    void searchDocuments_relevance_returns_empty_when_offset_exceeds_maxInt() {
+        // offset = pageNumber * pageSize; choose values so offset > Integer.MAX_VALUE
+        Pageable hugePage = org.springframework.data.domain.PageRequest.of(Integer.MAX_VALUE / 10 + 1, 10);
+
+        DocumentSearchResult result = documentService.searchDocuments(
+                "Brief", null, null, null, null, null, null, null,
+                DocumentSort.RELEVANCE, null, null, hugePage);
+
+        assertThat(result.items()).isEmpty();
+        verify(documentRepository, never()).findFtsPageRaw(anyString(), anyInt(), anyInt());
+    }
+
+    // ─── toFtsPage — UUID-as-String JDBC driver variance ────────────────────
+
+    @Test
+    void searchDocuments_relevance_handles_string_uuid_from_jdbc_driver() {
+        String stringId = "11111111-1111-1111-1111-111111111111";
+        UUID uuidId = UUID.fromString(stringId);
+        // Simulate a JDBC driver that returns the id column as String instead of UUID
+        List<Object[]> ftsRows = new ArrayList<>();
+        ftsRows.add(new Object[]{stringId, 0.5d, 1L});
+        when(documentRepository.findFtsPageRaw(anyString(), anyInt(), anyInt())).thenReturn(ftsRows);
+        when(documentRepository.findAllById(any())).thenReturn(List.of(doc(uuidId)));
+
+        DocumentSearchResult result = documentService.searchDocuments(
+                "Brief", null, null, null, null, null, null, null,
+                DocumentSort.RELEVANCE, null, null, PAGE);
+
+        assertThat(result.items()).hasSize(1);
+        assertThat(result.items().get(0).document().getId()).isEqualTo(uuidId);
+    }
+
     // ─── RELEVANCE sort — text + active filter ────────────────────────────────
 
     @Test