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/BookingEndpointTest.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\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Availability\AvailabilitySlot;
 use Unsupervised\Schedular\Booking\BookingEndpoint;
@@ -46,6 +47,12 @@ class BookingEndpointTest extends TestCase
         // Fixed "now" well before the fixture slot start (2026-07-01 10:00), so
         // the cancellation cutoff never trips unless a test moves it.
         Functions\when('current_time')->justReturn('2026-06-01 10:00:00');
+        // A successful booking resolves the student's name and the lesson date for
+        // the (mocked) instructor notice; neither shapes what these tests assert.
+        Functions\when('get_userdata')->justReturn(false);
+        Functions\when('mysql2date')->alias(
+            static fn (string $format, string $date): string => date($format, (int) strtotime($date))
+        );
 
         $this->availability = Mockery::mock(AvailabilityRepository::class);
         $this->bookings     = Mockery::mock(BookingRepository::class);
@@ -78,6 +85,11 @@ class BookingEndpointTest extends TestCase
         $this->sessions->shouldReceive('upcomingForStudent')->andReturn([])->byDefault();
         $this->sessions->shouldReceive('upcomingForInstructor')->andReturn([])->byDefault();
 
+        // 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->endpoint = new BookingEndpoint(
             $this->availability,
             $this->bookings,
@@ -87,7 +99,7 @@ class BookingEndpointTest extends TestCase
             // The real booker over the same mocked repositories: these tests are
             // about what a booking does end to end, and the booker is where most
             // of that now lives.
-            new LessonBooker($this->availability, $this->bookings, $this->offerings, $this->payments, $this->guardians),
+            new LessonBooker($this->availability, $this->bookings, $this->offerings, $this->payments, $this->guardians, $instructorMailer),
             new CancellationPolicy($this->settings),
             $this->guardians,
             $this->sessions,