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