fix(backend): round owed amounts via half-even policy in loanController.ts roundToCents (#1372)

[jotel-dev]

Fix: Use money policy half-even rounding in loanController roundToCents (#1372)

Description

Fixes issue #1372, where roundToCents in backend/src/controllers/loanController.ts previously used Math.floor, causing owed amounts and amortization portions to be systematically floored down (undercharging borrowers/protocol).

Replacing Math.floor with standard Math.round would have violated the repository's money policy (backend/src/money/policy.generated.ts), which defines half_even (banker's rounding) for settlement and balance calculations.

This PR delegates cent rounding to the central money module (backend/src/money/decimal.ts) using half_even tie-breaking.

Key Changes

• Money Module (backend/src/money/decimal.ts):
  • Added roundToCents(value: number, mode?: RoundingMode) helper which converts floating point values into stroops (toStroops) and divides by 100_000n using roundDiv under half_even mode.
• Loan Controller (backend/src/controllers/loanController.ts):
  • Replaced the local Math.floor implementation of roundToCents with the money policy's half_even roundToCents function.
• Unit Tests:
  • Added backend/src/__tests__/roundToCents.test.ts and updated backend/src/money/__tests__/decimal.test.ts to cover .5 tie-breaking half-even scenarios (e.g., 0.125 → 0.12, 0.135 → 0.14, 10.005 → 10.00, -0.125 → -0.12).
  • Fixed jest globals import in backend/src/__tests__/loanConfig.test.ts.

Pull Request Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation accordingly.
• I have verified the changes locally.

Verification

Executed local suite verifications:

npm run typecheck  # Passed cleanly with 0 errors
npm run lint       # ESLint passed cleanly
npm test           # All 53 test suites & 335 tests passed


closes #1372