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

[teeschima]

Hi,

I'd like to take on this issue.

From the requirements, the implementation appears to involve constructing a Soroban contract invocation that calls repay_installment() on the credit line contract, returning an unsigned XDR for wallet signing, and then implementing a separate submission flow that accepts the signed XDR, submits it to the network, waits for ledger confirmation, and returns the transaction hash.

My plan is to:

• Implement buildRepaymentXdr() in loans.service.ts
• Ensure the generated XDR invokes the correct contract method
• Add submitRepayment() in blockchain.service.ts
• Wait for transaction confirmation before returning success
• Update controller endpoints for the two-step signing flow
• Maintain strict TypeScript typing (no any)
• Verify npm run build passes before opening the PR
• Include testing notes and reference the issue in the PR

I have experience working with TypeScript backend services, API design, blockchain integrations, and transaction-signing workflows.

Looking forward to contributing.

[laxjovial]

Hi, I'd like to work on this issue.

I have experience with TypeScript backend development and blockchain integrations, including transaction construction, signing workflows, and smart contract interactions. For this task, I'll implement buildRepaymentXdr() to generate valid unsigned XDRs that invoke the repay_installment() function on the creditline contract, and add submitRepayment() to handle submission and ledger confirmation.

I'll ensure the two-step flow is implemented correctly, validate that the generated XDR targets the correct contract method, return transaction hashes on successful confirmation, maintain strict typing with no any usage, and verify the project builds successfully before submitting the PR.

Looking forward to contributing.

[deslawson]

I'd like to take on this issue. I have experience with Rust/Soroban smart contracts and will implement reentrancy guards across all mutating functions, add the necessary tests, and ensure cargo build and the full test suite pass before submitting a PR.

[divicbold47]

Hello maintainer, pls assign me this task

[grantfox-oss]

🦊 GrantFox — @deslawson has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #33)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@deslawson's PR #51 was approved and merged by @EmeditWeb.

🏆 @deslawson: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (363 total points). Track your full progress on GrantFox.

👏 Great work, @deslawson! Keep contributing to StepFi-app.