fix(document): tie-break equal-date DATE sort by title asc, not createdAt

Owner decision (#668): when two documents share a meta_date, order them by
title ascending instead of createdAt ascending. title is @Column(nullable=false)
so it is always present, giving a deterministic, human-meaningful total order.
Only the DATE-sort fast path changes; the in-memory SENDER/RECEIVER/RELEVANCE
comparators are untouched.

ORDER BY meta_date <dir> NULLS LAST, title ASC

Tests assert title-asc tiebreaking for same-date rows in BOTH directions, with a
fixture whose title order is the OPPOSITE of insertion (createdAt) order so the
test fails if the tiebreaker reverts to createdAt. The integration test drives
the production resolveSort against real Postgres.

Refs #668

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

@@ -839,11 +839,12 @@ public class DocumentService {
             // 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
+            // The title tiebreaker gives a stable total order when every row is
             // null-dated (the "Nur undatierte" filter), so pagination is deterministic.
+            // title is @Column(nullable=false), so it is always present.
             return Sort.by(
                     new Sort.Order(direction, "documentDate").nullsLast(),
-                    Sort.Order.asc("createdAt"));
+                    Sort.Order.asc("title"));
         }
         // SENDER and RECEIVER are sorted in-memory before this method is called
         return switch (sort) {