feat(document): add undatedOnly Specification for the undated-only filter

undatedOnly(false) is a no-op (null predicate); undatedOnly(true) returns
documentDate IS NULL, matching the existing hasStatus null-as-no-op pattern.
Real-Postgres tests pin the load-bearing guarantees H2 cannot prove: ASC
NULLS-LAST ordering, BETWEEN excludes null-dated rows, and that undated=true
combined with a from/to range returns empty (the collision rule).

Refs #668

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

@@ -55,6 +55,12 @@ public class DocumentSpecifications {
         return (root, query, cb) -> status == null ? null : cb.equal(root.get("status"), status);
     }
 
+    // Filtert auf undatierte Dokumente (meta_date IS NULL) — für die "Nur undatierte"-Triage.
+    // false → kein Prädikat (no-op), true → documentDate IS NULL (issue #668).
+    public static Specification<Document> undatedOnly(boolean undated) {
+        return (root, query, cb) -> undated ? cb.isNull(root.get("documentDate")) : null;
+    }
+
     /**
      * Filtert nach vorausgeweiteten Tag-ID-Sets mit AND- oder OR-Logik.
      *