Fix: unlock returns 500 on expired challenge instead of 401

[barry01-hash]

Expired challenge tokens currently produce a 500. Return 401 with a clear 'request a new challenge' message.

[Marvelg256]

@Marvelg256 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi! I'd like to work on this issue. I've reviewed the requirements and believe I have a good understanding of what's needed. I'm confident I can implement a clean solution, test it thoroughly, and submit a PR within 48 hours. If assigned, I'll keep the issue updated with my progress.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Marvelg256 to this issue.

[cybermax4200]

@cybermax4200 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I’d like to work on this issue because it addresses an important area of the project and presents a great opportunity to make a meaningful contribution. I’m particularly interested in delivering a solution that is reliable, maintainable, and aligned with the project’s existing architecture and development standards.

I have experience working across software development, including backend and frontend development, APIs, databases, testing, DevOps, and blockchain-based applications. I’m comfortable understanding existing codebases, identifying the root cause of issues, implementing clean and scalable solutions, and writing thorough tests to prevent regressions.

My approach would be to first understand the existing implementation and project conventions, then develop the solution in a focused and incremental way. I’ll ensure the changes are well-tested, documented where necessary, and do not introduce unnecessary complexity or break existing functionality. I’m also committed to clear communication, responding to review feedback, and delivering a polished contribution that provides real value to the project.

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[DevFaster]

@DevFaster has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer,

I've run build` and verified everything locally.
I'd like to work on this issue and get it fixed properly.
Please assign this to me. Thanks!

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

[drips-wave]

Congratulations, @Marvelg256! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Marvelg256: 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.

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