feat: add AFC Store with MercadoPago purchases and prize redemption

Players can now buy AfterCoin with real money (MercadoPago Checkout Pro,
$15 MXN/AFC) and redeem AFC for gift cards or cash withdrawals. Admin
fulfills redemptions manually.

- Bridge: payments + redemptions tables, CRUD routes, PATCH auth
- Next.js API: verify-disk, balance, create-preference, webhook (idempotent
  minting with HMAC signature verification), redeem, payment/redemption history
- Frontend: hub, buy flow (4 packages + custom), redeem flow (gift cards +
  cash out), success/failure/pending pages, history with tabs, 8 components
- i18n: full English + Spanish translations
- Infra: nginx /api/afc/ → Next.js, docker-compose env vars, .env.example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/.env.example

@@ -30,3 +30,10 @@ CF_API_TOKEN=your_cloudflare_api_token
 AFC_ADMIN_ADDRESS=0xYOUR_ADMIN_ADDRESS
 AFC_ADMIN_PRIVATE_KEY=your_private_key_without_0x_prefix
 AFC_BRIDGE_SECRET=change_me_in_production
+
+# AFC Store (MercadoPago integration)
+MERCADOPAGO_ACCESS_TOKEN=your_mp_access_token
+MERCADOPAGO_WEBHOOK_SECRET=your_mp_webhook_secret
+MERCADOPAGO_WEBHOOK_URL=https://yourdomain.com/api/afc/webhook
+AFC_PRICE_MXN=15
+NEXT_PUBLIC_SITE_URL=http://localhost:3000