Add unlisted event status: hidden from listings but accessible by URL

- Backend: add 'unlisted' to schema enum and Zod validation; allow booking for unlisted events
- Frontend: Event type and guards updated; unlisted events bookable, excluded from public listing/sitemap
- Admin: badge, status dropdown, Make Unlisted / Make Public / Unpublish actions; scanner/emails/tickets include unlisted

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/admin/emails/page.tsx

@@ -568,7 +568,7 @@ export default function AdminEmailsPage() {
                   className="w-full px-4 py-3 rounded-btn border border-secondary-light-gray"
                 >
                   <option value="">Choose an event</option>
-                  {events.filter(e => e.status === 'published').map((event) => (
+                  {events.filter(e => e.status === 'published' || e.status === 'unlisted').map((event) => (
                     <option key={event.id} value={event.id}>
                       {event.title} - {new Date(event.startDatetime).toLocaleDateString(locale === 'es' ? 'es-ES' : 'en-US', { timeZone: 'America/Asuncion' })}
                     </option>