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>

src/AdminMenu.php

@@ -10,6 +10,8 @@ use Unsupervised\Schedular\Auth\RegistrationController;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Booking\LessonController;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
+use Unsupervised\Schedular\GroupClass\GroupClassController;
 use Unsupervised\Schedular\Offering\OfferingController;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Policy\PolicyController;
@@ -27,14 +29,16 @@ class AdminMenu {
 	private QuestionController $questionController;
 	private PolicyController $policyController;
 	private RegistrationController $registrationController;
+	private GroupClassController $groupClassController;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites, EnrollmentRepository $enrollments ) {
 		$this->availabilityController = new AvailabilityController( $availability, $offerings );
 		$this->lessonController       = new LessonController( $bookings );
 		$this->offeringController     = new OfferingController( $offerings );
 		$this->questionController     = new QuestionController( $questions, $offerings );
 		$this->policyController       = new PolicyController( $policies, $policyVersions, $policyService );
 		$this->registrationController = new RegistrationController( $invites );
+		$this->groupClassController   = new GroupClassController( $enrollments, $offerings );
 	}
 
 	public function register(): void {
@@ -107,6 +111,17 @@ class AdminMenu {
 			35
 		);
 
+		// Studio admin: all group-class enrolments.
+		add_menu_page(
+			__( 'Group Classes', 'unsupervised-schedular' ),
+			__( 'Group Classes', 'unsupervised-schedular' ),
+			RoleManager::CAP_VIEW_ALL_LESSONS,
+			'us-group-classes',
+			[ $this->groupClassController, 'renderPage' ],
+			'dashicons-groups',
+			36
+		);
+
 		// Instructor: view their upcoming lessons.
 		add_menu_page(
 			__( 'My Lessons', 'unsupervised-schedular' ),