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/lib/api.ts

@@ -516,7 +516,7 @@ export interface Event {
   price: number;
   currency: string;
   capacity: number;
-  status: 'draft' | 'published' | 'cancelled' | 'completed' | 'archived';
+  status: 'draft' | 'published' | 'unlisted' | 'cancelled' | 'completed' | 'archived';
   bannerUrl?: string;
   externalBookingEnabled?: boolean;
   externalBookingUrl?: string;