fix: restore cross-platform binary builds

backend/internal/bootstrap/systemd_socket_nonlinux.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package bootstrap
+
+import "errors"
+
+func systemdSocket() (*socket, error) {
+	return nil, errors.New("systemd socket activation is only supported on Linux")
+}