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/components/Settings/NotificationsTab.tsx

@@ -25,6 +25,7 @@ const EVENT_LABEL_KEYS: Record<string, string> = {
   trip_invite: 'settings.notifyTripInvite',
   booking_change: 'settings.notifyBookingChange',
   trip_reminder: 'settings.notifyTripReminder',
+  todo_due: 'settings.notifyTodoDue',
   vacay_invite: 'settings.notifyVacayInvite',
   photos_shared: 'settings.notifyPhotosShared',
   collab_message: 'settings.notifyCollabMessage',