OpenAPI enriched spec (live-link #27)

[YaronZaki]

• See Missing /health endpoint — CI readiness check will always timeout #27. This issue is documentation hygiene only: a Markdown-rendered
  copy of the OpenAPI spec is committed to /docs/api/openapi.md for
  offline review.
• Files / Modules Affected
  • docs/api/openapi.md

[Davichi-1]

Hello! I have experience in full-stack and blockchain development, and I'd like to take on this issue. I can provide a clean, maintainable solution. Please assign it to me if it's still open.

[grantfox-oss]

🦊 GrantFox — @Davichi-1 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #296)
2. Your PR will be reviewed by the Quantarq maintainers

Good luck! Track your progress on GrantFox.