feat(notifications): reminders for todos with upcoming due dates

Todos already support a due_date field but nothing notifies the user
when a deadline is approaching — you'd only remember if you happened
to look at the Lists tab. This wires a reminder into the existing
notification pipeline so due-date todos behave like trip-start
reminders.

Details:
- New `todo_due` event type alongside trip_reminder; all four channels
  (in-app, email, webhook, ntfy) supported and toggleable per user in
  Settings > Notifications.
- New daily scheduler task (9 AM local TZ) queries unchecked todos
  whose due_date is within the next 3 days. Each todo gets at most
  one reminder per 24 hours, tracked via a new todo_items.reminded_at
  column (migration 116).
- If the todo has an assigned user, only that user is reminded; if
  not, every member of the trip gets the notification.
- Strings added in all 15 UI languages and for all notification
  carriers.
- Gated by app_settings.notify_todo_due (default on) so admins can
  disable it globally.

client/src/i18n/translations/br.ts

@@ -199,6 +199,7 @@ const br: Record<string, string | { name: string; category: string }[]> = {
   'settings.notifyTripInvite': 'Convites de viagem',
   'settings.notifyBookingChange': 'Alterações de reserva',
   'settings.notifyTripReminder': 'Lembretes de viagem',
+  'settings.notifyTodoDue': 'Tarefa com vencimento',
   'settings.notifyVacayInvite': 'Convites de fusão Vacay',
   'settings.notifyPhotosShared': 'Fotos compartilhadas (Immich)',
   'settings.notifyCollabMessage': 'Mensagens de chat (Colab)',
@@ -1935,6 +1936,8 @@ const br: Record<string, string | { name: string; category: string }[]> = {
   'notif.booking_change.text': '{actor} atualizou uma reserva em {trip}',
   'notif.trip_reminder.title': 'Lembrete de viagem',
   'notif.trip_reminder.text': 'Sua viagem {trip} está chegando!',
+  'notif.todo_due.title': 'Tarefa com vencimento',
+  'notif.todo_due.text': '{todo} em {trip} vence em {due}',
   'notif.vacay_invite.title': 'Convite Vacay Fusion',
   'notif.vacay_invite.text': '{actor} convidou você para fundir planos de férias',
   'notif.photos_shared.title': 'Fotos compartilhadas',