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/Auth/InstructorNotificationPrefTest.php

@@ -0,0 +1,64 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Auth;
+
+use Brain\Monkey\Functions;
+use Unsupervised\Schedular\Auth\InstructorNotificationPref;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class InstructorNotificationPrefTest extends TestCase
+{
+    public function testDefaultsOffWhenTheMetaIsUnset(): void
+    {
+        // Absent meta reads as an empty string, which is not the opt-in value.
+        Functions\when('get_user_meta')->justReturn('');
+
+        self::assertFalse((new InstructorNotificationPref())->wants(7));
+    }
+
+    public function testReadsAStoredOptIn(): void
+    {
+        Functions\when('get_user_meta')->alias(
+            static fn (int $id, string $key): string => 7 === $id && InstructorNotificationPref::META_NOTIFY === $key ? '1' : ''
+        );
+
+        self::assertTrue((new InstructorNotificationPref())->wants(7));
+    }
+
+    public function testAStoredNoReadsAsOff(): void
+    {
+        Functions\when('get_user_meta')->justReturn('0');
+
+        self::assertFalse((new InstructorNotificationPref())->wants(7));
+    }
+
+    public function testNobodyWantsNothing(): void
+    {
+        // A zero id is not a user; it must never read as opted-in.
+        Functions\expect('get_user_meta')->never();
+
+        self::assertFalse((new InstructorNotificationPref())->wants(0));
+    }
+
+    public function testSetStoresTheChoiceAsAStringBoolean(): void
+    {
+        Functions\expect('update_user_meta')->once()->with(7, InstructorNotificationPref::META_NOTIFY, '1')->andReturn(true);
+
+        (new InstructorNotificationPref())->set(7, true);
+    }
+
+    public function testSetStoresADeliberateNoRatherThanDeleting(): void
+    {
+        Functions\expect('update_user_meta')->once()->with(7, InstructorNotificationPref::META_NOTIFY, '0')->andReturn(true);
+
+        (new InstructorNotificationPref())->set(7, false);
+    }
+
+    public function testSetIgnoresANonUser(): void
+    {
+        Functions\expect('update_user_meta')->never();
+
+        (new InstructorNotificationPref())->set(0, true);
+    }
+}