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/Offering.php

@@ -42,6 +42,7 @@ class Offering {
 		public readonly ?string $termEnd = null,
 		public readonly ?string $scheduleNote = null,
 		public readonly ?string $etransferEmail = null,
+		public readonly ?int $cancellationCutoffHours = null,
 		public readonly bool $isActive = true,
 		public readonly ?int $id = null,
 	) {}
@@ -83,6 +84,7 @@ class Offering {
 			termEnd:         Val::stringOrNull( $row->term_end ),
 			scheduleNote:    Val::stringOrNull( $row->schedule_note ),
 			etransferEmail:  Val::stringOrNull( $row->etransfer_email ),
+			cancellationCutoffHours: Val::intOrNull( $row->cancellation_cutoff_hours ),
 			isActive:        Val::bool( $row->is_active ),
 			id:              Val::int( $row->id ),
 		);
@@ -99,21 +101,22 @@ class Offering {
 	 */
 	public function toArray( bool $includeEtransferEmail = true ): array {
 		$out = [
-			'id'               => $this->id,
-			'instructor_id'    => $this->instructorId,
-			'kind'             => $this->kind,
-			'title'            => $this->title,
-			'description'      => $this->description,
-			'duration_minutes' => $this->durationMinutes,
-			'price'            => $this->price,
-			'currency'         => $this->currency,
-			'billing_mode'     => $this->billingMode,
-			'allow_weekly'     => $this->allowWeekly,
-			'capacity'         => $this->capacity,
-			'term_start'       => $this->termStart,
-			'term_end'         => $this->termEnd,
-			'schedule_note'    => $this->scheduleNote,
-			'is_active'        => $this->isActive,
+			'id'                        => $this->id,
+			'instructor_id'             => $this->instructorId,
+			'kind'                      => $this->kind,
+			'title'                     => $this->title,
+			'description'               => $this->description,
+			'duration_minutes'          => $this->durationMinutes,
+			'price'                     => $this->price,
+			'currency'                  => $this->currency,
+			'billing_mode'              => $this->billingMode,
+			'allow_weekly'              => $this->allowWeekly,
+			'capacity'                  => $this->capacity,
+			'term_start'                => $this->termStart,
+			'term_end'                  => $this->termEnd,
+			'schedule_note'             => $this->scheduleNote,
+			'cancellation_cutoff_hours' => $this->cancellationCutoffHours,
+			'is_active'                 => $this->isActive,
 		];
 
 		if ( $includeEtransferEmail ) {