Admin trade management:
- GET /api/admin/trades/upcoming: Upcoming booked trades (sorted by time)
- GET /api/admin/trades/past: Past trades with filters
- status, start_date, end_date, user_search
- POST /api/admin/trades/{id}/complete: Mark as completed (after slot time)
- POST /api/admin/trades/{id}/no-show: Mark as no-show (after slot time)
- POST /api/admin/trades/{id}/cancel: Admin cancel trade
AdminExchangeResponse includes user contact info for admin view.