Security: Implement Rate Limiting for Payment Endpoints

[gabito1451]

To prevent brute-forcing booking_ids or spamming the Stellar network, the payment endpoints need rate limiting.

Acceptance Criteria:

• Use slowapi or similar to limit /prepare, /submit, /release.
• Configure lower limits for failed transaction submissions.

[Kaycee276]

@Kaycee276 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I'd like to take this on — I have hands-on experience with Python/FastAPI and security hardening, and slowapi is a natural fit here as a drop-in rate limiter for FastAPI that I've worked with before. I'll apply limits to /prepare, /submit, and /release, with tighter thresholds specifically for failed transaction submissions to cut off brute-force attempts on booking_ids without penalizing legitimate traffic. I'll make sure limits are configurable and the implementation is tested for both the happy path and rate-exceeded scenarios.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kaycee276 to this issue.

[drips-wave]

Congratulations, @Kaycee276! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Kaycee276: 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 🌊