Function build_router 

pub fn build_router(state: AppState) -> Router

Assembles every route module into one [axum::Router], ready to serve or to drive directly in a test via tower::ServiceExt::oneshot.

Alongside the JSON API routes, this mounts the OpenAPI contract itself two ways: the raw document at GET /api/openapi.json (for tooling – CI’s spec-diff gate, and the generated frontend/ TS client) and an interactive Scalar UI at /api/docs (for a human exploring the API in a browser). [utoipa_scalar::Scalar::with_url] returns a state-generic axum::Router<S> with the UI’s one route already attached, so it merges in directly rather than needing its own handler function.

Everything that isn’t one of those routes falls through to the SPA fallback handler, which serves the built React SPA – so this one router is the whole HTTP surface of a real bhtune-server deployment, API and UI alike. None of the merged sub-routers set their own fallback (axum panics if two merged routers each declare one), so this is the only place .fallback is called.