Add invoice creation deposit slashing for spam prevention

[Kingsman-99]

Label: complexity: high
Points: 200

Description

creation_fee goes straight to treasury and is never returned, but there's no refundable deposit mechanism that gets slashed only on bad behavior (e.g. creating an invoice that's cancelled within minutes). This issue adds an optional refundable anti-spam deposit.

Technical Context

Involves lib.rs — set_spam_deposit(env, admin, amount: i128, min_age_secs: u64); create_invoice() charges the deposit into contract custody; cancel_invoice() returns it only if now - created_at >= min_age_secs, otherwise it's slashed to treasury.

Acceptance Criteria

• Deposit is charged in addition to creation_fee, held by the contract (not treasury) until resolved
• Cancelling after min_age_secs returns the full deposit to the creator
• Cancelling before min_age_secs sends the deposit to treasury instead
• Deposit of 0 (default) disables the feature entirely
• Test: early cancel slashes deposit to treasury, late cancel refunds creator
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[johnsmccain]

@johnsmccain has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would like to work on this

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[grantfox-oss]

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Congratulations, @johnsmccain! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@johnsmccain's PR #247 was approved and merged by @Kingsman-99.

🏆 @johnsmccain: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (258 total points). Track your full progress on GrantFox.

👏 Great work, @johnsmccain! Keep contributing to Stellar-split.

[drips-wave]

This issue has been completed by @johnsmccain as part of the Stellar Wave Program's 6th Wave 🥳

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