Switch instructor availability from blacklist to whitelist

Request: Switch instructor scheduling from blacklisting unavailability to whitelisting availability, and add an automatic buffer that removes unbooked slots across both online and front-desk booking.

1. Whitelist availability

Let instructors mark when they are available rather than blocking off when they're not. 

2. Automatic booking buffer

Add a configurable buffer (e.g. 36 hours) that automatically removes an available slot once it's inside the buffer window and still unbooked — applied to both online and front-desk booking. The existing online booking window doesn't stop front-desk staff from booking inside the cutoff; we enforce that with internal policy today, but a system-level rule is far more reliable.

Configurability: Facility-wide default with a per-instructor override.