Devlog #16: Internal API schemas

Quedula has an internal HTTP API between the frontend and backend. Maintaining it without documentation means creating debt that accumulates.

We use an OpenAPI schema as the single source of truth for the API contract. The schema is generated from code — this ensures it doesn't drift from reality.

A frontend client is automatically generated from the schema. This means changing a field type on the backend immediately shows up as a type error on the frontend — before deployment.

An extra bonus: the schema can be used to generate test data and to validate requests at the middleware level.