sql: add desc index on snh(operation_id)

sql/schema.sql

@@ -311,4 +311,6 @@ begin
     user_id = new.user_id,
     news_id = new.news_id,
     "date" = new.date;
-end;
+end;
+
+create index if not exists idx_snh_operation_id_desc on squadrons_news_historical (operation_id desc);