/// R4: reward_declaration must be created within phase.started_at + 7 days.
bool validateRewardDeclarationWindow({
  required String phaseStartedAt, // YYYY-MM-DD
  required DateTime now,
}) {
  final start = DateTime.parse(phaseStartedAt);
  final cutoff = start.add(const Duration(days: 7));
  return !now.isAfter(cutoff);
}