feat(domain,presentation,tests): implement Relay entity, DTOs, and API errors

- Add Relay entity with constructors and business logic methods
- Add RelayDto for API responses with From<Relay> conversion
- Add ApiError with ResponseError trait for unified error handling
- Add dependency injection tests for mock infrastructure in test mode

nix/shell.nix

@@ -38,7 +38,7 @@ inputs.devenv.lib.mkShell {
         nodePackages.pnpm
       ];
 
-      processes.run.exec = "bacon run";
+      processes.backend-run.exec = "bacon run";
 
       enterShell = ''
         echo "🦀 Rust MCP development environment loaded!"