Implement two-step repayment flow with buildRepaymentXdr() and submit…

[deslawson]

🔗 Related Issue

Closes #33

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🔖 Title

Implement two-step repayment flow with buildRepaymentXdr() and submitRepayment()

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📝 Description

Adds a two-step loan repayment flow that replaces the broken repay_loan contract call with the correct repay_installment() function. Step 1 — Build unsigned XDR (POST /loans/:loanId/pay):

• Validates loan ownership, active status, and payment amount against remaining balance
• Calls repay_installment() on the creditline contract via CreditLineContractClient.buildRepayLoanTx()
• Returns unsigned XDR + payment preview Step 2 — Submit signed XDR (POST /loans/:loanId/repay):
• Accepts a signed XDR in the request body
• Submits to Horizon and polls for ledger confirmation (up to ~60s)
• Returns { transactionHash } on success New files: src/modules/blockchain/blockchain.module.ts, src/modules/blockchain/blockchain.service.ts Modified files: creditline.client.ts, loans.service.ts, loans.controller.ts, loans.module.ts, app.module.ts

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🔄 Changes Made

• I Fixed contract function name — Changed creditline.client.ts to call repay_installment() instead of the non-existent repay_loan()
• Added buildRepaymentXdr() — New method in loans.service.ts that validates loan state (ownership, active status, balance) and returns an unsigned XDR for repay_installment()
• Added submitRepayment() — New method in blockchain.service.ts that submits a signed XDR to Horizon and polls for ledger confirmation before returning the transaction hash
• Added POST /loans/:loanId/repay endpoint — New controller endpoint accepting { xdr: string } to complete the two-step flow
• Created BlockchainModule — New self-contained module exporting BlockchainService, imported by LoansModule and AppModule

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📸 Screenshots (if applicable)

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🗒️ Additional Notes

Closed #33

[EmeditWeb]

@deslawson fix CI issues