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/admin/group-classes.php

@@ -0,0 +1,36 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+/** @var list<array{student: string, offering: string, status: string}> $rows */
+?>
+<div class="wrap">
+    <h1><?php esc_html_e('Group Classes', 'unsupervised-schedular'); ?></h1>
+    <p class="description"><?php esc_html_e('Active enrolments across all group classes.', 'unsupervised-schedular'); ?></p>
+
+    <?php if (empty($rows)) : ?>
+        <p><?php esc_html_e('No active enrolments.', 'unsupervised-schedular'); ?></p>
+    <?php else : ?>
+        <table class="wp-list-table widefat fixed striped">
+            <thead>
+                <tr>
+                    <th><?php esc_html_e('Student', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Class', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Status', 'unsupervised-schedular'); ?></th>
+                </tr>
+            </thead>
+            <tbody>
+                <?php foreach ($rows as $row) : ?>
+                    <tr>
+                        <td><?php echo esc_html($row['student']); ?></td>
+                        <td><?php echo esc_html($row['offering']); ?></td>
+                        <td><?php echo esc_html($row['status']); ?></td>
+                    </tr>
+                <?php endforeach; ?>
+            </tbody>
+        </table>
+    <?php endif; ?>
+</div>