[Feature] Add per-installment late fee accrual

[EmeditWeb]

Problem

The RepaymentInstallment struct in contracts/creditline-contract/src/types.rs carries no due_date, and repay_installment() does not penalize late payments. Borrowers can effectively pay months late with zero consequence, which breaks the reputation score's underlying assumption that on-time payment matters.

Context

Late fees are the protocol's only economic enforcement mechanism short of liquidation. Without them, sponsors price all risk into the base APR, hurting on-time learners. The fee must flow back into the liquidity pool so sponsors are made whole.

Before Starting

Read these context files first:

• context/architecture-context.md
• context/code-standards.md
• context/progress-tracker.md
• contracts/creditline-contract/src/types.rs
• contracts/creditline-contract/src/lib.rs

What To Build

1. Add pub due_date: u64 to RepaymentInstallment in types.rs. Populate at loan creation: due_date = approval_ts + (n+1) * installment_period_secs.
2. Add pub late_fee_bps: u32 to ProtocolParameters in types.rs, default 500 bps (5%).
3. In repay_installment(), compute now = env.ledger().timestamp(). If now > installment.due_date, compute late_fee = installment.amount * late_fee_bps / 10_000 and require the caller to transfer installment.amount + late_fee.
4. Route the late fee to the liquidity pool via cross-contract call, not the borrower's loan balance.
5. Emit a LATEFEEPAID event with (loan_id, installment_index, fee_amount).
6. Add 4 unit tests: on-time path (no fee), 1-day-late, exact-due-date boundary, custom-bps test.

Files To Touch

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

Acceptance Criteria

• due_date is set on every installment at loan creation
• late_fee_bps is configurable and persisted in ProtocolParameters
• Late fees route to the liquidity pool, not the loan balance
• On-time payments incur zero fee (exact now == due_date is on-time)
• 4 new tests pass
• Migration note added: existing loans without due_date are gracefully handled (assume 0 = on-time)

Mandatory Checks Before PR

• cargo build passes with zero errors
• cargo test — all 93 existing tests still pass
• require_auth() is FIRST line of every mutating function
• extend_ttl() called after EVERY persistent storage write
• New unit tests written for every new function
• context/progress-tracker.md updated

[aweledavid3]

Hi Maintainer,

I would like to contribute to this project.

I am a full stack developer with a passion for software development, open-source collaboration, and artificial intelligence. I have experience working with JavaScript, Python, C++, and Rust, with a primary focus on frontend development using React, Next.js, and TypeScript.

[EmmanuelAdah]

I'd be glad to take up this issue. Leveraging my backend engineering background and specific experience developing Rust smart contracts on the Soroban engine, I can efficiently implement this late fee enforcement mechanism. I will update the relevant structs to track due dates and the configurable late fee, and modify repay_installment to calculate and route any accrued penalties directly to the liquidity pool via a cross-contract call. Finally, I will ensure backward compatibility for existing loans, emit the LATEFEEPAID event, and implement the four boundary tests to guarantee flawless fee logic before updating the progress tracker.

[Salauayo]

I’ll implement installment due dates and configurable late fees, routing penalties directly to the liquidity pool while preserving the loan balance. I’ll add the LATEFEEPAID event, handle existing loans safely, update storage/TTL requirements, and add tests for on-time, boundary, late, and custom-fee cases. I’ll also run the full build/test suite and update the progress tracker before PR.
kindly assign to me

[bobo-24]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. I’ll prioritize this immediately and ensure a reliable solution is delivered as soon as possible. Thank you!

[Peculiarr7]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[afeezorobsco-cyber]

i would like to work on this issue

[grantfox-oss]

🦊 GrantFox — @afeezorobsco-cyber has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #99)
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 Third Campaign campaign!

@afeezorobsco-cyber's PR #101 was approved and merged by @EmeditWeb.

🏆 @afeezorobsco-cyber: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (659 total points). Track your full progress on GrantFox.

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