Availability API

Read-only endpoints used by the booking page. Creating a booking still requires a logged-in learner and a server-side conflict check.

GET /api/coverage?postcode=SW16+2AA&transmission=automatic
GET /api/slots?postcode=SW16+2AA&transmission=automatic&service_id=1&date=2026-09-08

Times in slot payloads are ISO-8601 UTC. Display them in Europe/London. POST /webhooks/payments/{provider} accepts provider callbacks; duplicate webhook ids are ignored.