authenticate and query availability and reservations via the bookingkit api v3

Steps

1. confirm bookingkit and Regiondo are separate, independently branded companies with separate developer docs — bookingkit was not part of the 2023 Regiondo/Rezdy/Checkfront merger, so do not assume unified branding or a shared API
2. obtain a BusinessKit account and request API client credentials (client_id/client_secret) by contacting bookingkit support, since there is no self-serve key generation
3. request an OAuth 2.0 access token using the Client Credentials grant against bookingkit's token endpoint with grant_type=client_credentials, client_id, and client_secret; tokens expire after 3600 seconds with no refresh-token flow — just request a new one
4. include the token as a Bearer value in the Authorization header on requests to the API v3 base at api.bookingkit.de
5. call calendar/event endpoints with the calendar_read scope to check availability, and use orders_read_owned / orders_write_owned (or the _all variants for marketplace access) scopes to list, create, or update reservations against the events and orders endpoints
6. use the staging environment (api-stage.bookingkit.de) for development/testing before switching to production