setStartDateBackBy method
Implementation
Future<void> setStartDateBackBy({required int days}) async {
await deleteProgress();
progress = await SupabaseQuery.batchUpsert<SubjectProgress>(
progress.map((p) => p.setStartDateBackBy(days: days).toJson()).toList(),
);
startedAt = startedAt!.subtract(Duration(days: days));
save(onlyUpdate: true);
}