/// Delay before retry number [attempt] (1-based): [base] doubled per attempt,
/// capped at [max].
Duration exponentialBackoff(
  int attempt, {
  Duration base = const Duration(milliseconds: 500),
  Duration max = const Duration(seconds: 5),
}) {
  final exponent = (attempt - 1).clamp(0, 30);
  final millis = base.inMilliseconds * (1 << exponent);
  return millis >= max.inMilliseconds ? max : Duration(milliseconds: millis);
}