core: implement interest accrual calculation on-chain

[EmeditWeb]

Problem

Interest rate calculation currently happens off-chain
in the API. This means the API can charge any interest
rate regardless of what the parameters and reputation
contracts say.

What To Build

1. On-chain interest calculation in create_loan()
2. Store monthly_payment, interest_rate_bps, total_repayment
3. Update repay_installment() to use stored values

Files To Touch

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

Acceptance Criteria

• Interest calculated from on-chain reputation score
• Correct rate applied per tier
• monthly_payment stored in loan on creation
• repay_installment uses stored monthly_payment
• Tests cover all 4 reputation tiers

Mandatory Checks Before PR

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