fix(db): total ordering for series OFFSET pagination

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).

internal/database/querier.go

@@ -221,6 +221,11 @@ type Querier interface {
 	// Series are grouped PER LIBRARY: the same series name in two libraries
 	// (novels vs manga adaptations, LN splits) stays two entries. Visible
 	// filter enforces per-user library visibility at the source.
+	// Tiebreaker REQUIRED for LIMIT/OFFSET pagination: bulk imports in one
+	// transaction share a single NOW() timestamp, leaving many groups tied on
+	// MAX(created_at). Postgres does not guarantee tie order between
+	// executions, so page boundaries could repeat or drop series. The GROUP BY
+	// keys make the sort total and deterministic.
 	GetDistinctSeries(ctx context.Context, arg GetDistinctSeriesParams) ([]GetDistinctSeriesRow, error)
 	GetDistinctSeriesCount(ctx context.Context, arg GetDistinctSeriesCountParams) (int32, error)
 	GetFailedSyncQueueItems(ctx context.Context, limit int32) ([]SyncQueue, error)