curl -X POST "https://olive-gateway-a6ba.onrender.com/api/v1/cards/link" \
  -H "Authorization: Bearer olive_live_xxx" \
  -H "Content-Type: application/json" \
  -d '{
    "subscriber_id": "sub_abc123",
    "card_serial": "OLIV0001",
    "pin": "1234"
  }'

{
  "success": true,
  "message": "Card linked successfully",
  "card": {
    "serial": "OLIV0001",
    "mac_address": "AA:BB:CC:DD:EE:01",
    "type": "PARENT",
    "status": "ACTIVE",
    "subscriber_id": "sub_abc123",
    "linked_at": "2025-01-15T10:30:00Z"
  }
}