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>

templates/frontend/group-classes-page.php

@@ -0,0 +1,16 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+?>
+<div id="us-group-app">
+    <div id="us-group-list">
+        <p><?php esc_html_e('Loading group classes…', 'unsupervised-schedular'); ?></p>
+    </div>
+    <div id="us-group-confirmation" style="display:none;">
+        <p><?php esc_html_e('You are enrolled. The studio will be in touch.', 'unsupervised-schedular'); ?></p>
+    </div>
+    <div id="us-group-error" style="display:none;" role="alert"></div>
+</div>