fix

module.nix

@@ -167,7 +167,7 @@ in
           POSTGRES_USER = db.user;
           POSTGRES_HOST = db.host;
           USERDATA_FILE_PATH = "/var/lib/mastodon/.userdata.json";
-          OWNER_USERNAME = db.username;
+          OWNER_USERNAME = sp.username;
         };
         serviceConfig = {
           Slice = "mastodon.slice";