Criar cupom
POST /api/v1/subscriptions/coupons — requer X-Client-ID + X-API-Key.
Resposta
201:
Atualizar cupom
PUT /api/v1/subscriptions/coupons/{uuid} — requer X-Client-ID + X-API-Key.
Campos editáveis: name, description, max_redemptions, max_cycles, valid_until, is_active.
code, discount_type e discount_value são imutáveis após a criação.