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/Booking/CancellationPolicy.php

@@ -0,0 +1,69 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Booking;
+
+use Unsupervised\Schedular\Payment\StudioSettings;
+
+/**
+ * Decides whether a student may still cancel a lesson. Cancellation closes once
+ * the lesson starts within the effective cutoff window; instructors and studio
+ * admins bypass this entirely and cancel through other paths.
+ *
+ * The window is resolved per lesson: the offering's own cutoff when it sets one,
+ * otherwise the studio default. Both are expressed in hours.
+ */
+class CancellationPolicy {
+
+	public function __construct( private StudioSettings $settings ) {}
+
+	/**
+	 * The effective cutoff in hours for a lesson: the offering's override when
+	 * set (a non-negative value), otherwise the studio default.
+	 */
+	public function cutoffHours( ?int $offeringCutoffHours ): int {
+		if ( null !== $offeringCutoffHours && $offeringCutoffHours >= 0 ) {
+			return $offeringCutoffHours;
+		}
+
+		return $this->settings->cancellationCutoffHours();
+	}
+
+	/**
+	 * Whether a student may still cancel a lesson starting at $slotStartDt
+	 * (WordPress-local `Y-m-d H:i:s`), given the offering's optional cutoff
+	 * override. A zero cutoff always allows cancellation; unparseable input
+	 * fails open so a student is never trapped by bad data. Pass $now to make
+	 * the comparison deterministic in tests.
+	 */
+	public function studentMayCancel( string $slotStartDt, ?int $offeringCutoffHours, ?string $now = null ): bool {
+		$hours = $this->cutoffHours( $offeringCutoffHours );
+		if ( $hours <= 0 ) {
+			return true;
+		}
+
+		$start   = strtotime( $slotStartDt );
+		$current = strtotime( $now ?? current_time( 'mysql' ) );
+		if ( false === $start || false === $current ) {
+			return true;
+		}
+
+		return ( $start - $current ) >= $hours * 3600;
+	}
+
+	/**
+	 * A human-readable description of a cutoff for student-facing messages:
+	 * whole days as days, anything else as hours.
+	 */
+	public function describeCutoff( int $hours ): string {
+		if ( $hours > 0 && 0 === $hours % 24 ) {
+			$days = $hours / 24;
+
+			/* translators: %d: number of days. */
+			return sprintf( _n( '%d day', '%d days', $days, 'unsupervised-schedular' ), $days );
+		}
+
+		/* translators: %d: number of hours. */
+		return sprintf( _n( '%d hour', '%d hours', $hours, 'unsupervised-schedular' ), $hours );
+	}
+}