feat: integrate RollyPay payment provider (SBP via USDT)
Full integration following PayPear/SeverPay patterns:
- API client with X-API-Key + X-Nonce auth, HMAC-SHA256 webhook verification
- RollyPayPaymentMixin with create, webhook, finalize (all side effects), status check
- CRUD, model, migration (0059), PaymentMethod.ROLLYPAY enum
- Webhook endpoint, cabinet topup, bot handler
- Statuses: created, processing, paid, expired, canceled, chargeback
- Config: ROLLYPAY_ENABLED, ROLLYPAY_API_KEY, ROLLYPAY_SIGNING_SECRET, etc.
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