core: implement installment repayment XDR builder

[EmeditWeb]

Problem

POST /loans/:id/repay exists but the service does not
build the actual unsigned XDR calling repay_installment()
on the creditline contract.

What To Build

1. Implement buildRepaymentXdr() in loans.service.ts
2. Add submitRepayment() in blockchain.service.ts
3. Two-step flow: get unsigned XDR, then submit signed

Files To Touch

• src/modules/loans/loans.service.ts
• src/modules/loans/loans.controller.ts
• src/modules/blockchain/blockchain.service.ts

Acceptance Criteria

• buildRepaymentXdr() returns valid XDR
• XDR calls correct contract function
• Submission waits for ledger confirmation
• Transaction hash returned on success
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue