feat: add OpenAPI spec (#9)

Claude added an OpenAPI spec for the Spade API. It allows a Swagger
preview to be generated for inspecting, learning and trying out the API.

<img width="981" height="623" alt="Screenshot 2026-01-23 at 14 52 32"
src="https://github.com/user-attachments/assets/906d418a-21bd-4df3-a883-cb947d41b486"
/>

Author: alanshaw