fix: there's no ALTER TABLE ... ADD INDEX in postgresql

SQL/postgres.initial.sql

@@ -131,7 +131,7 @@ CREATE TABLE "cache" (
     data text NOT NULL
 );
 
-ALTER TABLE "cache" ADD INDEX (user_id, cache_key);
+CREATE INDEX cache_user_id_idx ON "cache" (user_id, cache_key);
 
 --
 -- Sequence "message_ids"