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/RestRegistrar.php

@@ -7,6 +7,8 @@ use Unsupervised\Schedular\Availability\AvailabilityEndpoint;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingEndpoint;
 use Unsupervised\Schedular\Booking\BookingRepository;
+use Unsupervised\Schedular\GroupClass\EnrollmentEndpoint;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
 use Unsupervised\Schedular\Offering\OfferingEndpoint;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Policy\PolicyEndpoint;
@@ -26,13 +28,15 @@ class RestRegistrar {
 	private OfferingEndpoint $offeringEndpoint;
 	private QuestionEndpoint $questionEndpoint;
 	private PolicyEndpoint $policyEndpoint;
+	private EnrollmentEndpoint $enrollmentEndpoint;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, RegistrationGate $gate ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, RegistrationGate $gate, EnrollmentRepository $enrollments ) {
 		$this->availabilityEndpoint = new AvailabilityEndpoint( $availability );
 		$this->bookingEndpoint      = new BookingEndpoint( $availability, $bookings, $offerings, $gate );
 		$this->offeringEndpoint     = new OfferingEndpoint( $offerings );
 		$this->questionEndpoint     = new QuestionEndpoint( $questions, $offerings );
 		$this->policyEndpoint       = new PolicyEndpoint( $policies, $policyVersions, $policyService );
+		$this->enrollmentEndpoint   = new EnrollmentEndpoint( $enrollments, $offerings, $gate );
 	}
 
 	public function register(): void {
@@ -45,5 +49,6 @@ class RestRegistrar {
 		$this->offeringEndpoint->registerRoutes( self::NAMESPACE );
 		$this->questionEndpoint->registerRoutes( self::NAMESPACE );
 		$this->policyEndpoint->registerRoutes( self::NAMESPACE );
+		$this->enrollmentEndpoint->registerRoutes( self::NAMESPACE );
 	}
 }