feat: Add OpenAPI / Swagger documentation for all REST API endpoints

[Haroldwonder]

Context

The backend and currency API have no machine-readable API specification. API.md and FEE_SERVICE_API.md are hand-written and may drift from the implementation. External integrators have no way to generate client SDKs.

Expected Behavior

An OpenAPI 3.0 spec (openapi.yaml) should document all endpoints in both api/ and backend/ services, including request/response schemas, error codes, and authentication requirements.

Suggested Solution

Use zod-to-openapi or tsoa to generate the spec from TypeScript types, keeping it in sync automatically.

Acceptance Criteria

• openapi.yaml covers all endpoints
• Spec is validated with swagger-cli validate
• GET /api/docs serves Swagger UI
• CI fails if spec is out of date

Priority: Medium

[Zarmaijemimah]

@Zarmaijemimah has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I can handle this issue and I will love to work on it please kindly assign thank you.

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

[drips-wave]

Congratulations, @Zarmaijemimah! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @Zarmaijemimah: 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 marked as completed by a Drips Wave moderator for @Zarmaijemimah as part of the Stellar Wave Program's 3rd Wave 🥳

Moderator's reason: [Auto-assessed] The contributor successfully implemented the requested OpenAPI documentation for both the API and backend services. The PR includes comprehensive YAML specifications, Swagger UI integration, automated validation scripts, and a CI/CD workflow that satisfies all acceptance criteria.

😎 @Zarmaijemimah: 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.

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

ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.