P2E: add claimable reward ledger

[ilyar]

Source PRD: tasks/prd-full-play-to-earn.md
Parent epic: #86
PRD section: US-006

Description

Earned rewards need explicit claimable records with transparent status and idempotency.

Acceptance Criteria

• Add claimable reward records with reward ID, account ID, campaign ID, pool ID, reward type, gross amount, net amount, currency/unit, status, expiration, and idempotency key.
• Status values include pending, eligibility_review, claimable, claim_started, paid, rejected, expired, and canceled.
• Claimable rewards are generated only from finalized campaign results or approved instant-reward rules.
• Duplicate generation cannot create duplicate rewards for the same account/campaign/rule.
• Reward records are append-only or have an event trail for all status changes.
• Tests cover reward generation, idempotency, status transitions, expiration, and cancellation.

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

Reward claim status changes must be auditable.