GetDistinctSeries ordered by MAX(created_at) DESC alone. Bulk imports
inserted in a single transaction share one NOW() timestamp, leaving
large numbers of (library_id, series) groups tied on MAX(created_at);
Postgres does not guarantee tie order between executions, so
LIMIT/OFFSET pages could repeat or drop series across page loads.
Add the GROUP BY keys (library_id, series) as tiebreakers so the sort
is total and deterministic. Comment documents why the tiebreaker is
required so it is not dropped as noise. sqlc regenerated (interface
unchanged; the comment flows into querier.go docs).