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/zh.ts

@@ -199,6 +199,7 @@ const zh: Record<string, string> = {
   'settings.notifyTripInvite': '旅行邀请',
   'settings.notifyBookingChange': '预订变更',
   'settings.notifyTripReminder': '旅行提醒',
+  'settings.notifyTodoDue': '待办事项即将到期',
   'settings.notifyVacayInvite': 'Vacay 融合邀请',
   'settings.notifyPhotosShared': '共享照片 (Immich)',
   'settings.notifyCollabMessage': '聊天消息 (Collab)',
@@ -1936,6 +1937,8 @@ const zh: Record<string, string> = {
   'notif.booking_change.text': '{actor} 更新了 {trip} 中的预订',
   'notif.trip_reminder.title': '旅行提醒',
   'notif.trip_reminder.text': '您的旅行 {trip} 即将开始！',
+  'notif.todo_due.title': '待办事项即将到期',
+  'notif.todo_due.text': '{trip} 中的 {todo} 将于 {due} 到期',
   'notif.vacay_invite.title': 'Vacay 融合邀请',
   'notif.vacay_invite.text': '{actor} 邀请您合并假期计划',
   'notif.photos_shared.title': '照片已分享',