defiant: Enable postgresql backups

hosts/defiant/services/hedgedoc.nix

@@ -95,6 +95,8 @@ in {
     }];
   };
 
+  services.postgresqlBackup.databases = [ "hedgedoc" ];
+
   services.nginx.virtualHosts."${domain}" = {
     listen = [
       { addr = "192.168.10.175"; port = 43443; ssl = true; }