Notify instructors on new bookings and enrolments

Add an opt-in, per-instructor email notice sent when someone books one of
their lessons or enrols in one of their group classes. Off by default and
set from My Availability → Notifications; covers both the student/guardian
REST flows and the studio's wp-admin "book/add for a student" forms.

The opt-in check lives in InstructorNotificationMailer so no booking path
can drift on who is mailed; lessons fire from LessonBooker::settle (the one
step both booking paths reach), enrolments from EnrollmentEndpoint::enroll
and GroupClassController::addDirect. The notice is a courtesy and never
fails a booking or enrolment that otherwise succeeded.

Co-authored-by: anthropic/claude-opus-4-8

src/Auth/InstructorNotificationPref.php

@@ -0,0 +1,54 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+use Unsupervised\Schedular\Val;
+
+/**
+ * An instructor's own choice of whether the studio emails them when someone
+ * enrols in one of their group classes or books one of their lessons.
+ *
+ * It is a per-instructor preference, kept in user meta rather than a studio-wide
+ * option, because the people it emails are the ones who should decide: one
+ * instructor wants a heads-up for every booking, another already lives in the
+ * roster and wants no extra mail. The instructor sets it from their own
+ * {@see \Unsupervised\Schedular\Availability\AvailabilityController My
+ * Availability} page.
+ *
+ * It defaults **off**. The notification is a new capability, and turning it on
+ * for every instructor on an existing site the day it ships would mail people
+ * who never asked to be mailed; an instructor who wants it opts in.
+ */
+class InstructorNotificationPref {
+
+	/**
+	 * User-meta key holding the per-instructor toggle. Stored as `'1'` / `'0'`,
+	 * mirroring the `us_*` string-boolean convention the plugin's options use.
+	 */
+	public const META_NOTIFY = 'us_notify_on_booking';
+
+	/**
+	 * Whether the given instructor has asked to be emailed about new enrolments
+	 * and bookings. Absent meta — the default for every account — reads as off.
+	 */
+	public function wants( int $instructorId ): bool {
+		if ( $instructorId <= 0 ) {
+			return false;
+		}
+
+		return '1' === Val::string( get_user_meta( $instructorId, self::META_NOTIFY, true ) );
+	}
+
+	/**
+	 * Record an instructor's choice. Stored as `'0'` rather than deleted so a
+	 * deliberate "no" is told apart from an account that never chose.
+	 */
+	public function set( int $instructorId, bool $wants ): void {
+		if ( $instructorId <= 0 ) {
+			return;
+		}
+
+		update_user_meta( $instructorId, self::META_NOTIFY, $wants ? '1' : '0' );
+	}
+}