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/admin/events/[id]/page.tsx

@@ -619,7 +619,7 @@ export default function AdminEventDetailPage() {
                 <div>
                   <p className="font-medium">Capacity</p>
                   <p className="text-gray-600">{confirmedCount + checkedInCount} / {event.capacity} spots filled</p>
-                  <p className="text-sm text-gray-500">{event.capacity - confirmedCount - checkedInCount} spots remaining</p>
+                  <p className="text-sm text-gray-500">{Math.max(0, event.capacity - confirmedCount - checkedInCount)} spots remaining</p>
                 </div>
               </div>
             </div>