Add cancellation cutoff limiting how close to a lesson a student can cancel

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]>

src/Offering/OfferingController.php

@@ -104,6 +104,11 @@ class OfferingController {
 		$duration = absint( Val::int( $_POST['duration_minutes'] ?? 0 ) );
 		$capacity = absint( Val::int( $_POST['capacity'] ?? 0 ) );
 
+		// A blank cutoff means "use the studio default" (null); any entered value
+		// (including 0 — cancel any time) is a per-offering override.
+		$cutoffRaw   = trim( sanitize_text_field( Val::string( wp_unslash( $_POST['cancellation_cutoff_hours'] ?? '' ) ) ) );
+		$cutoffHours = '' === $cutoffRaw ? null : absint( Val::int( $cutoffRaw ) );
+
 		// Term dates: a class either meets once (term ends the day it starts)
 		// or repeats weekly for a set number of sessions.
 		$termStart = Offering::normalizeDate( sanitize_text_field( Val::string( wp_unslash( $_POST['term_start'] ?? '' ) ) ) );
@@ -129,6 +134,7 @@ class OfferingController {
 			termEnd:         $termEnd,
 			scheduleNote:    $this->nullableText( sanitize_text_field( Val::string( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ) ),
 			etransferEmail:  $this->nullableText( sanitize_email( Val::string( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ) ),
+			cancellationCutoffHours: $cutoffHours,
 			isActive:        isset( $_POST['is_active'] ),
 			id:              $existing?->id,
 		);