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

templates/admin/availability.php

@@ -15,6 +15,7 @@ if (! defined('ABSPATH')) {
  * @var string $nextWeek
  * @var string $notice Success message from the submitted action; empty when none.
  * @var string $error  Failure message from the submitted action; empty when none.
+ * @var bool   $notifyOnBooking Whether the instructor is emailed on new bookings/enrolments.
  */
 
 use Unsupervised\Schedular\Availability\AvailabilitySlot;
@@ -44,6 +45,25 @@ $deleteForm = static function (\Unsupervised\Schedular\Availability\Availability
         <div class="notice notice-error is-dismissible"><p><?php echo esc_html($error); ?></p></div>
     <?php endif; ?>
 
+    <h2><?php esc_html_e('Notifications', 'unsupervised-schedular'); ?></h2>
+    <form method="post">
+        <?php wp_nonce_field('usc_availability_action'); ?>
+        <input type="hidden" name="usc_action" value="save_notify">
+        <table class="form-table">
+            <tr>
+                <th scope="row"><?php esc_html_e('Booking emails', 'unsupervised-schedular'); ?></th>
+                <td>
+                    <label>
+                        <input type="checkbox" name="notify_on_booking" value="1" <?php checked($notifyOnBooking); ?>>
+                        <?php esc_html_e('Email me when someone books a lesson or enrols in one of my group classes', 'unsupervised-schedular'); ?>
+                    </label>
+                    <p class="description"><?php esc_html_e('Off by default. Covers both your private lessons and your group classes, whether the student booked themselves or the studio booked for them.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
+        </table>
+        <?php submit_button(esc_html__('Save Notification Preference', 'unsupervised-schedular')); ?>
+    </form>
+
     <h2><?php esc_html_e('Add Availability', 'unsupervised-schedular'); ?></h2>
     <p><?php esc_html_e('The window must start and end on the same day. It is split into bookable slots of the chosen lesson length — for example, 9:00 AM–4:00 PM with 60-minute lessons creates seven slots.', 'unsupervised-schedular'); ?></p>
     <form method="post" id="usc-add-availability">