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/GroupClass/GroupClassControllerTest.php

@@ -5,6 +5,7 @@ namespace Unsupervised\Schedular\Tests\Unit\GroupClass;
 
 use Brain\Monkey\Functions;
 use Mockery;
+use Unsupervised\Schedular\Auth\InstructorNotificationMailer;
 use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\RegistrationMailer;
 use Unsupervised\Schedular\Auth\RoleManager;
@@ -32,6 +33,7 @@ class GroupClassControllerTest extends TestCase
     private RegistrationMailer&Mockery\MockInterface $mailer;
     private IntakeAudit&Mockery\MockInterface $audit;
     private IntakeRecording&Mockery\MockInterface $intake;
+    private InstructorNotificationMailer&Mockery\MockInterface $instructorMailer;
     private GroupClassController $controller;
 
     protected function setUp(): void
@@ -47,6 +49,10 @@ class GroupClassControllerTest extends TestCase
         $this->mailer         = Mockery::mock(RegistrationMailer::class);
         $this->audit          = Mockery::mock(IntakeAudit::class);
         $this->intake         = Mockery::mock(IntakeRecording::class);
+        // The opt-in instructor notice is tested on its own; a mock keeps these
+        // tests about enrolment, not about who gets emailed.
+        $this->instructorMailer = Mockery::mock(InstructorNotificationMailer::class);
+        $this->instructorMailer->shouldReceive('notifyEnrollment')->andReturn(true)->byDefault();
         $this->controller     = new GroupClassController(
             $this->enrollments,
             $this->offerings,
@@ -57,6 +63,7 @@ class GroupClassControllerTest extends TestCase
             $this->mailer,
             $this->audit,
             $this->intake,
+            $this->instructorMailer,
         );
 
         Functions\when('current_user_can')->justReturn(true);