P2E: add payout provider abstraction

[ilyar]

Source PRD: tasks/prd-full-play-to-earn.md
Parent epic: #86
PRD section: US-007

Description

Payout rails must sit behind backend adapters so rewards are not hardcoded to one provider or currency.

Acceptance Criteria

• Define RewardPayoutProvider interface for quote, create payout, status sync, cancel where supported, and provider webhook handling.
• Supported provider types are config-driven: manual, gift_card, ton_wallet, crypto_exchange, fiat_provider, or telegram_if_supported.
• MVP can use manual provider for testnet or controlled private payouts.
• Production provider requires secrets only on backend, never in frontend.
• Payout provider errors are mapped to stable internal error codes.
• Tests use a fake provider and cover success, pending, failure, retry, duplicate webhook, and provider outage.

Constraints

• Keep current paid PvP settlement as entry fee, not player-funded prize pool.
• Do not make Season Points redeemable or refundable.
• Keep payout/compliance/provider secrets backend-only.
• Treat this as deferred future-phase work until mechanics/self-host production readiness is stable.

Notes

Any blockchain payout path needs separate legal approval and wallet-risk controls.