Add NixOS VM integration tests and update CI/CD pipeline configuration. Introduce checks for backend integration, full-stack, performance, and security tests using native NixOS services. Remove legacy GitHub Actions workflow and replace with Gitea Actions runner configuration. Update README and quickstart guide to reflect new development environment setup and testing commands.

flake.nix

@@ -129,6 +129,9 @@
             '';
           };
         };
+
+        # NixOS VM tests
+        checks = import ./nixos/tests.nix { inherit pkgs; };
       }
     );
 }