Add group-class enrolment (year commitment, capacity, registration gate)

Implements #4: students enrol in a group_class offering via the same
registration gate as private lessons (intake questions + booking-scoped
policy acceptance). Enrolment is capacity-enforced and prevents duplicates.

- Schema: us_group_enrollments table.
- Enrollment value object + EnrollmentRepository (countActiveForOffering,
  hasActiveEnrollment, per-student/instructor/all-active queries, status).
- EnrollmentEndpoint: GET /enrollments (scoped) and POST /enrollments
  (validates group_class, capacity, no-duplicate; reuses RegistrationGate;
  records answers/acceptances type enrollment).
- GroupClassController + admin page (view_all_lessons): all active enrolments.
- Front-end: [us_group_classes] shortcode (GroupClassPage) + group-classes.js
  enrol flow (list classes -> questions + policies -> POST /enrollments).
- Wiring in Plugin, RestRegistrar, AdminMenu, ShortcodeRegistrar.

Payment is the deferred seam (#7): enrolment lands active, payment_id null.
JS left untested for parity with the repo's no-build vanilla-JS posture.

Tests: tests/Unit/GroupClass/ (Enrollment, EnrollmentRepository).
composer test (121), cs, and PHPStan level 6 all pass.

Refs #4

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/GroupClass/EnrollmentTest.php

@@ -0,0 +1,52 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\GroupClass;
+
+use Unsupervised\Schedular\GroupClass\Enrollment;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class EnrollmentTest extends TestCase
+{
+    public function testDefaults(): void
+    {
+        $enrollment = new Enrollment(offeringId: 7, studentId: 5, instructorId: 3);
+
+        self::assertSame(Enrollment::STATUS_ACTIVE, $enrollment->status);
+        self::assertNull($enrollment->paymentId);
+        self::assertNull($enrollment->id);
+    }
+
+    public function testStatusConstants(): void
+    {
+        self::assertContains(Enrollment::STATUS_ACTIVE, Enrollment::VALID_STATUSES);
+        self::assertContains(Enrollment::STATUS_CANCELLED, Enrollment::VALID_STATUSES);
+        self::assertContains(Enrollment::STATUS_COMPLETED, Enrollment::VALID_STATUSES);
+    }
+
+    public function testFromRowMapsCorrectly(): void
+    {
+        $enrollment = Enrollment::fromRow((object) [
+            'id'            => '12',
+            'offering_id'   => '7',
+            'student_id'    => '5',
+            'instructor_id' => '3',
+            'status'        => Enrollment::STATUS_ACTIVE,
+            'payment_id'    => null,
+        ]);
+
+        self::assertSame(12, $enrollment->id);
+        self::assertSame(7, $enrollment->offeringId);
+        self::assertSame(5, $enrollment->studentId);
+        self::assertNull($enrollment->paymentId);
+    }
+
+    public function testToArrayContainsExpectedKeys(): void
+    {
+        $arr = (new Enrollment(7, 5, 3, id: 12))->toArray();
+
+        foreach (['id', 'offering_id', 'student_id', 'instructor_id', 'status', 'payment_id'] as $key) {
+            self::assertArrayHasKey($key, $arr);
+        }
+    }
+}