Machine-readable OpenAPI contract for the full API.

FastAPI auto-generates the schema from every registered route (21 paths: search, sources, resolve, document, cite, report-issue, collections, deep, …). The default /openapi.json live route can be shadowed by a hosting catch-all, so this exposes the same spec at a stable, agent-addressable path. app.openapi() is cached by FastAPI after first build.