module: let systemd handle statedirectory

module.nix

···
         WorkingDirectory = cfg.repo.scanPath;
         ExecStart = lib.getExe tangledPkgs.knotserver;
         Restart = "always";
+
+        StateDirectory = mkIf (lib.hasPrefix "/var/lib/tangled-knotserver" cfg.repo.scanPath) "tangled-knotserver";
         EnvironmentFile = cfg.environmentFile;
+        # TODO: hardening
       };
 
       environment = {