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/db/migrations.ts

@@ -1792,6 +1792,13 @@ function runMigrations(db: Database.Database): void {
         CREATE INDEX IF NOT EXISTS idx_prt_hash ON password_reset_tokens(token_hash);
       `);
     },
+    // Migration: todo due-date reminders — track when we last sent a
+    // reminder for each todo so we don't spam the same notification
+    // every day the scheduler runs.
+    () => {
+      try { db.exec('ALTER TABLE todo_items ADD COLUMN reminded_at DATETIME'); }
+      catch (err: any) { if (!err.message?.includes('duplicate column name')) throw err; }
+    },
   ];
 
   if (currentVersion < migrations.length) {