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/GroupClass/GroupClassPage.php

@@ -0,0 +1,36 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\GroupClass;
+
+use Unsupervised\Schedular\Auth\RoleManager;
+
+class GroupClassPage {
+
+	/**
+	 * Renders the group-class enrolment shortcode output.
+	 *
+	 * @param array<string, string> $atts Shortcode attributes (unused — reserved for future options).
+	 */
+	public function render( array $atts ): string { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found
+		if ( ! is_user_logged_in() ) {
+			return sprintf(
+				'<p>%s <a href="%s">%s</a>.</p>',
+				esc_html__( 'Please', 'unsupervised-schedular' ),
+				esc_url( wp_login_url( get_permalink() ) ),
+				esc_html__( 'log in to enrol in a class', 'unsupervised-schedular' )
+			);
+		}
+
+		if ( ! current_user_can( RoleManager::CAP_BOOK_LESSON ) ) {
+			return '<p>' . esc_html__( 'This page is for students only.', 'unsupervised-schedular' ) . '</p>';
+		}
+
+		wp_enqueue_style( 'us-scheduler' );
+		wp_enqueue_script( 'us-scheduler-group' );
+
+		ob_start();
+		include USC_PLUGIN_DIR . 'templates/frontend/group-classes-page.php';
+		return (string) ob_get_clean();
+	}
+}