P2E: add seasons and reward campaigns

[ilyar]

Source PRD: tasks/prd-full-play-to-earn.md
Parent epic: #86
PRD section: US-003

Description

Rewards need visible seasons/campaigns so P2E has a bounded competitive structure.

Acceptance Criteria

• Add season/campaign records with ID, title, start time, end time, status, eligible game modes, eligible balance kinds, reward pool ID, and reward rules version.
• Only active seasons/campaigns can accrue reward points.
• Ended seasons stop accruing points but remain visible in history.
• Admin can create a draft campaign and preview reward math before activation.
• Campaign activation requires a funded reward pool and legal gate approval flag.
• Tests cover active, upcoming, ended, and canceled campaign states.

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

Campaign rules should be versioned and immutable after activation except pause/cancel paths.