[backend] Fix Loan Repayment Rounding: Amounts Always Rounded Down

[ExcelDsigN-tech]

[backend] Fix Loan Repayment Rounding: Amounts Always Rounded Down

Description: Loan repayment calculations in backend/src/services/loanController.ts use Math.floor() causing systematic underpayment. Borrowers pay less than owed; lenders lose interest over time.

Impact: Financial discrepancy accumulates across all loans; regulatory compliance issue; trust erosion.

Suggested Fix:

1. Replace Math.floor() with banker's rounding (Math.round()) or configurable rounding mode
2. Add rounding policy constant in backend/src/config/money.ts
3. Write property-based tests for rounding edge cases
4. Create migration to recalculate outstanding balances
5. Add audit log for rounding adjustments

Points: 200
Type: bug

Definition of Done

• Bug reproduced and root cause identified
• Fix implemented with configurable rounding policy
• Property-based tests cover edge cases (0.5, negative, large values)
• Migration script recalculates outstanding balances
• Audit logging for rounding adjustments
• No regression introduced
• All necessary CI checks passed

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[Masked18]

@Masked18 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer
Please can you assign me this issue

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[obedojukwu50-stack]

@obedojukwu50-stack has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I will like to work on this project, I know exactly what to do and I will deliver efficiently.

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[ekomdavid86-cell]

@ekomdavid86-cell has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

i would like to work on this issue because accurate financial calculations are critical for building trust and reliability in a lending platform

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[KingFRANKHOOD]

@KingFRANKHOOD has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

i will love to work on this issue

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