Add system user and group for webref in NixOS tests. Update database accessibility check to use webref user for improved security testing.

nixos/tests.nix

@@ -158,6 +158,13 @@
           }];
         };
         
+        # Create system user for testing
+        users.users.webref = {
+          isSystemUser = true;
+          group = "webref";
+        };
+        users.groups.webref = {};
+        
         environment.systemPackages = with pkgs; [
           python3
           nmap
@@ -169,7 +176,7 @@
       start_all()
       machine.wait_for_unit("postgresql.service")
       
-      # Verify database is accessible locally
+      # Verify database is accessible with webref user
       machine.succeed("sudo -u webref psql webref -c 'SELECT 1;'")
       
       machine.succeed("echo '✅ Security test passed'")