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

tests/Unit/Booking/AdminBookingTest.php

@@ -5,6 +5,7 @@ namespace Unsupervised\Schedular\Tests\Unit\Booking;
 
 use Brain\Monkey\Functions;
 use Mockery;
+use Unsupervised\Schedular\Auth\InstructorNotificationMailer;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Availability\AvailabilitySlot;
@@ -59,10 +60,15 @@ class AdminBookingTest extends TestCase
 
         // The real booker over mocked repositories: an admin booking must go
         // through exactly the machinery a student's own booking does.
+        // The opt-in instructor notice is tested on its own; a mock keeps these
+        // tests about booking, not about who gets emailed.
+        $instructorMailer = Mockery::mock(InstructorNotificationMailer::class);
+        $instructorMailer->shouldReceive('notifyLessonBooked')->andReturn(true)->byDefault();
+
         $this->admin = new AdminBooking(
             $this->availability,
             $this->offerings,
-            new LessonBooker($this->availability, $this->bookings, $this->offerings, $this->payments, $this->guardians)
+            new LessonBooker($this->availability, $this->bookings, $this->offerings, $this->payments, $this->guardians, $instructorMailer)
         );
     }