Students can no longer cancel their own lesson online once it starts within a
configured window; instructors and studio admins can always cancel.
- Studio default `us_cancellation_cutoff_hours` (stored/computed in hours,
entered and displayed in days under Studio Settings → Cancellations).
- Optional per-offering override `cancellation_cutoff_hours` (entered in hours);
blank inherits the studio default, 0 allows anytime cancellation.
- `Booking\CancellationPolicy` resolves the effective window and decides;
`BookingEndpoint::cancel()` returns a 403 `cancellation_closed` when too late.
The instructor status endpoint and studio-admin student actions bypass it.
Closes#93
Co-Authored-By: Claude Opus 4.8 <[email protected]>
