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/Plugin.php

@@ -7,6 +7,7 @@ use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingRepository;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Policy\AcceptanceRepository;
 use Unsupervised\Schedular\Policy\PolicyRepository;
@@ -32,11 +33,12 @@ class Plugin {
 		$policyService    = new PolicyService( $policies, $policyVersions );
 		$acceptances      = new AcceptanceRepository( $wpdb );
 		$invites          = new InviteRepository( $wpdb );
+		$enrollments      = new EnrollmentRepository( $wpdb );
 		$registrationGate = new RegistrationGate( $questions, $answers, $policies, $policyVersions, $acceptances );
 
 		( new RoleManager() )->register();
-		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites ) )->register();
-		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate ) )->register();
+		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites, $enrollments ) )->register();
+		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate, $enrollments ) )->register();
 		( new ShortcodeRegistrar( $invites, $policies, $policyVersions, $acceptances ) )->register();
 	}
 }