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/page.tsx

@@ -140,7 +140,7 @@ export default function EventsPage() {
                           <div className="flex items-center gap-2">
                             <UserGroupIcon className="w-4 h-4" />
                             <span>
-                              {event.availableSeats} / {event.capacity} {t('events.details.spotsLeft')}
+                              {Math.max(0, event.capacity - (event.bookedCount ?? 0))} / {event.capacity} {t('events.details.spotsLeft')}
                             </span>
                           </div>
                         )}