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.
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