core: implement loan installment due date tracking

[EmeditWeb]

Problem

RepaymentInstallment has no due_date field.
The protocol cannot determine if a payment is
on-time or late.

What To Build

1. Add due_date to RepaymentInstallment struct
2. Set due dates in approve_loan()
3. Record paid_at timestamp
4. Add is_on_time() helper

Files To Touch

• contracts/creditline-contract/src/types.rs
• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/tests.rs

Acceptance Criteria

• All installments have due dates set on approval
• paid_at recorded on repayment
• is_on_time() returns correct result
• Reputation updated based on payment timing

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[Osifowora]

kindly assign

[Oluwatos94]

Hi @EmeditWeb,

I'd like to take on this issue. Having worked on this project. I am very conversant with the code base, and I can confidently deliver the expected result.

Ready to start immediately; looking forward to being assigned.

[grantfox-oss]

🦊 GrantFox — @Oluwatos94 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #22)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Oluwatos94's PR #41 was approved and merged by @EmeditWeb.

🏆 @Oluwatos94: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (226 total points). Track your full progress on GrantFox.

👏 Great work, @Oluwatos94! Keep contributing to StepFi-app.