Contracts: Add rewards points expiry — time-limited balances with TTL per user

[CelestinaBeing]

Summary

Unredeemed rewards accumulate indefinitely. For campaigns with promotional points (e.g. 'earn double points this month'), points should optionally expire after a configurable period. This is standard in loyalty programs and prevents indefinite liability for campaign operators.

Problem

• No expiry mechanism exists; balances are permanent
• Operators have no way to create time-limited promotional rewards
• Dormant balances inflate total_credited stats

Acceptance Criteria

• Add per-credit expiry: credit_with_expiry(env, from, user, amount, expires_at_ledger: u64) function
• Store per-user expiry map: (EXPIRY, user) -> Vec<(amount, expires_at)>
• balance(user) must filter expired credits before returning the sum
• claim() must consume non-expired credits first (FIFO)
• Add prune_expired(user) callable by anyone to clean up expired storage and free TTL (incentivize cleanup via no-op success)
• Add credit_no_expiry() alias for the current credit() behavior (backward compatible)
• Add unit tests: credit expires at correct ledger, balance returns 0 after expiry, claim uses non-expired first
• Document expiry mechanics in contracts/rewards/README.md

References

• contracts/rewards/src/lib.rs:180 — credit()
• contracts/rewards/src/lib.rs:264 — claim()

[michaelvic123]

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[Malcolm-Wander]

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Hello maintainer,I’d love to work on this issue…thank you in advance🙏🏽

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[Temi-suwa18]

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I would like to work to on this issue

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