core: implement vendor payment escrow in creditline

[EmeditWeb]

Problem

When a loan is approved, there is no mechanism to
actually pay the vendor. The vendor never receives
funds.

What To Build

1. Add vendor_paid flag to Loan struct
2. Add pay_vendor() function
3. Integrate with approve_loan() flow

Files To Touch

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

Acceptance Criteria

• Vendor receives loan amount on approval
• vendor_paid flag prevents double payment
• Event emitted with payment details
• Default scenario handled correctly
• Token transfer uses stored token contract

Mandatory Checks Before PR

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

[Iwayemi-Kehinde]

I would like to work on this issue as I have reviewed the scope, I should have a PR Ready in 50 minutes.

[divicbold47]

Hello maintainer, pls assign me this task