refactor(search): remove dead SENDER case from resolveSort switch

SENDER and RECEIVER are handled by in-memory sort before resolveSort
is called, making those switch cases unreachable. Removed and added
a comment making the invariant explicit.

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

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

@@ -308,9 +308,9 @@ public class DocumentService {
         if (sort == null || sort == DocumentSort.DATE) {
             return Sort.by(direction, "documentDate");
         }
+        // SENDER and RECEIVER are sorted in-memory before this method is called
         return switch (sort) {
             case TITLE -> Sort.by(direction, "title");
-            case SENDER -> Sort.by(direction, "sender.lastName").and(Sort.by(direction, "sender.firstName"));
             case UPLOAD_DATE -> Sort.by(direction, "createdAt");
             default -> Sort.by(direction, "documentDate");
         };