Add overflow protection to claimable amount calculation

[ogazboiz]

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Description

get_claimable_amount() multiplies rate_per_second * elapsed_seconds in i128. For very high-rate or long-duration streams this multiplication can overflow, returning a negative or incorrect value with no panic.

What Needs to Happen

• Replace raw * with checked_mul in the claimable calculation
• On overflow: cap at deposited_amount - withdrawn_amount
• Apply the same protection to claimable.service.ts in the backend (BigInt arithmetic)
• Add test: test_claimable_near_max_i128_no_overflow
• Add test: test_claimable_always_capped_at_remaining_balance

Files

• contracts/stream_contract/src/lib.rs (get_claimable_amount)
• backend/src/services/claimable.service.ts

Acceptance Criteria

• No panic or incorrect result for any valid i128 input
• Capped at deposited amount on overflow
• Both contract and service protected

[overprodigy]

@overprodigy has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I'd liek to work on this.

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Congratulations, @overprodigy! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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[drips-wave]

This issue has been completed by @overprodigy as part of the Stellar Wave Program's 4th Wave 🥳

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