Fix event capacity: no negative availability, sold-out enforcement, admin override

- Backend: use calculateAvailableSeats so availableSeats is never negative
- Backend: reject public booking when confirmed >= capacity; admin create/manual bypass capacity
- Frontend: spotsLeft = max(0, capacity - bookedCount), isSoldOut when bookedCount >= capacity
- Frontend: sold-out redirect on booking page, cap quantity by spotsLeft, never show negative

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

frontend/src/app/(public)/events/[id]/page.tsx

@@ -118,7 +118,7 @@ function generateEventJsonLd(event: Event) {
       '@type': 'Offer',
       price: event.price,
       priceCurrency: event.currency,
-      availability: event.availableSeats && event.availableSeats > 0
+      availability: Math.max(0, (event.capacity ?? 0) - (event.bookedCount ?? 0)) > 0
         ? 'https://schema.org/InStock'
         : 'https://schema.org/SoldOut',
       url: `${siteUrl}/events/${event.id}`,