Contracts: Add admin-configurable minimum claim amount in rewards contract

[CelestinaBeing]

Summary

Currently a user can claim as little as 1 point at a time. This creates spam risk on mainnet where each claim is a transaction with fees. A configurable minimum claim amount prevents micro-claims and keeps the fee-to-reward ratio sensible.

Problem

In contracts/rewards/src/lib.rs, the claim() function has no minimum amount check beyond 'amount > 0' (implicitly via u64). An attacker or buggy client can create thousands of 1-point claim transactions.

Acceptance Criteria

• Add MIN_CLAIM constant key to instance storage
• Add set_min_claim(env, admin, min_amount: u64) admin function (0 = no minimum)
• In claim(): return new Error::BelowMinClaim if amount < min_claim && min_claim > 0
• Add min_claim(env) -> u64 view function
• Add unit tests: claim below minimum rejected, claim exactly at minimum succeeds, admin can update minimum
• Emit min_claim_set event on admin update
• Update contracts/rewards/README.md

References

• contracts/rewards/src/lib.rs:264 — claim() function
• Error enum at contracts/rewards/src/lib.rs:20

[Valreb001]

@Valreb001 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello, please i would like to work on this issue, assign me please!!!

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[Temi-suwa18]

@Temi-suwa18 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to work to on this issue

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