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.

server/src/services/notificationService.ts

@@ -82,6 +82,13 @@ const EVENT_NOTIFICATION_CONFIG: Record<string, EventNotifConfig> = {
     navigateTextKey: 'notif.action.view_trip',
     navigateTarget: p => (p.tripId ? `/trips/${p.tripId}` : null),
   },
+  todo_due: {
+    inAppType: 'navigate',
+    titleKey: 'notif.todo_due.title',
+    textKey: 'notif.todo_due.text',
+    navigateTextKey: 'notif.action.view_trip',
+    navigateTarget: p => (p.tripId ? `/trips/${p.tripId}` : null),
+  },
   vacay_invite: {
     inAppType: 'navigate',
     titleKey: 'notif.vacay_invite.title',