[BE-29] Add resource credit balance endpoint

[yusuftomilola]

Overview

Add an endpoint that fetches a member's on-chain resource credit balance from the Soroban resource_credits contract. All implementation goes in backend/sandbox/.

Acceptance Criteria

• GET /sandbox/credits/balance returns the current user's credit balance
• Reads from the deployed Soroban resource_credits contract using the Stellar SDK
• Contract address is read from an env variable RESOURCE_CREDITS_CONTRACT_ID
• Returns { balance: string, address: string } (balance as string to avoid precision loss)
• Returns 503 with a clear message if the Stellar network is unreachable
• JWT-protected

[bytebinders]

@bytebinders has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I can implement the JWT-protected GET /sandbox/credits/balance endpoint to fetch the current user’s on-chain resource credit balance from the Soroban resource_credits contract using the Stellar SDK, with the contract address from the env variable and proper handling for network errors and response formatting.

Please assign this issue to Bytebinders. Thank you.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @bytebinders to this issue.

[ibdevlawal]

@ibdevlawal has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi, I’d like to contribute to this project by working on open issues. I have experience with javascript, and I’m comfortable collaborating, following guidelines, and submitting clean pull requests. Please let me know if there’s a good issue for me to start with. Thanks!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ibdevlawal to this issue.

[drips-wave]

Congratulations, @ibdevlawal! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @ibdevlawal: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @ibdevlawal as part of the Stellar Wave Program's 4th Wave 🥳

😎 @ibdevlawal: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.