Backend: Fix collateral valuation precision loss

[ONEONUORA]

Problem

Decimal calculations don't account for different token decimals.

Impact

• Precision loss in valuations
• Incorrect collateral calculations
• Financial inaccuracy

Solution

Implement proper decimal handling for different token precisions.

Files to Modify

• backend/src/risk_engine.rs

Priority

High - Financial accuracy

[RemmyAcee]

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

Based on your request, I will implement and fix this issue according to the design requirement and to fit your acceptance criteria

ℹ️ Repo Maintainers: To accept this application, review their application or assign @RemmyAcee to this issue.