Unchecked i128 Payout Multiplications

[hman38705]

Description
In high-inflation or large-pool scenarios, multiplying bet.amount * total_staked (from the planned fix in Issue 2/42) or related reward calculations could overflow i128. While i128 is massive, safe-math patterns are non-negotiable in financial smart contracts to prevent catastrophic accounting failures.

Requirements and context

• Use checked_mul and checked_div for all payout and stake calculations.
• Fail gracefully with clear error codes on overflow.

Suggested execution

• Fork the repo and create a branch: git checkout -b fix/issue-89-safe-math
• Globally refactor arithmetic in the rewards module.

Implementation changes

• Modify bets.rs (around the payout logic) to use Soroban's safe math utilities.

Test and commit

• Verify that extremely large pool sizes do not cause unexpected transaction failures.

Example commit message
fix: implement checked math for all pool-ratio payout calculations

Guidelines

• Arithmetic safety.
• Timeframe: 24 hours.