Remove MercadoPago as a direct payment option for services and promotions.
If the user has enough balance: show only "Pagar con saldo".
If not: show current balance, amount missing, and a "Recargar saldo" button
that leads to the recharge flow (where MercadoPago and Bre-B remain available).
Applies to both TelegramBotService (startPurchase, viewPromo) and
PublicChat (iniciarCompra, verPromocion).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
