fix(document): order undated documents last on the DATE sort fast path

resolveSort produced Sort.by(direction, "documentDate") with NATIVE null
handling, so Postgres surfaced undated (null meta_date) documents FIRST on
an ASC sort. Apply nullsLast() so undated rows order last for both ASC and
DESC, with a createdAt-asc tiebreaker for a stable total order when every
row is null-dated (the upcoming "Nur undatierte" filter).

Refs #668

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

@@ -797,10 +797,17 @@ public class DocumentService {
         return transcriptionBlockQueryService.getCompletionStats(docIds);
     }
 
-    private Sort resolveSort(DocumentSort sort, String dir) {
+    Sort resolveSort(DocumentSort sort, String dir) {
         Sort.Direction direction = "ASC".equalsIgnoreCase(dir) ? Sort.Direction.ASC : Sort.Direction.DESC;
         if (sort == null || sort == DocumentSort.DATE || sort == DocumentSort.RELEVANCE) {
-            return Sort.by(direction, "documentDate");
+            // Undated documents (null documentDate) must order last regardless of
+            // direction — Postgres puts NULLs FIRST on ASC by default, which would
+            // surface the undated pile at the top with no explanation (issue #668).
+            // The createdAt tiebreaker gives a stable total order when every row is
+            // null-dated (the "Nur undatierte" filter), so pagination is deterministic.
+            return Sort.by(
+                    new Sort.Order(direction, "documentDate").nullsLast(),
+                    Sort.Order.asc("createdAt"));
         }
         // SENDER and RECEIVER are sorted in-memory before this method is called
         return switch (sort) {