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>

tests/Unit/GroupClass/EnrollmentRepositoryTest.php

@@ -0,0 +1,113 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\GroupClass;
+
+use Brain\Monkey\Functions;
+use Mockery;
+use Unsupervised\Schedular\GroupClass\Enrollment;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class EnrollmentRepositoryTest extends TestCase
+{
+    private \wpdb $db;
+    private EnrollmentRepository $repo;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->db         = Mockery::mock(\wpdb::class);
+        $this->db->prefix = 'wp_';
+        $this->repo       = new EnrollmentRepository($this->db);
+    }
+
+    public function testInsertReturnsId(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-06-02 09:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_group_enrollments',
+                Mockery::on(static function (array $d): bool {
+                    return $d['offering_id'] === 7
+                        && $d['student_id'] === 5
+                        && $d['instructor_id'] === 3
+                        && $d['status'] === Enrollment::STATUS_ACTIVE;
+                }),
+                ['%d', '%d', '%d', '%s', '%d', '%s']
+            );
+        $this->db->insert_id = 12;
+
+        self::assertSame(12, $this->repo->insert(new Enrollment(7, 5, 3)));
+    }
+
+    public function testCountActiveForOffering(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/COUNT\(\*\).*offering_id = %d AND status = %s/s'), 7, Enrollment::STATUS_ACTIVE)
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_var')->andReturn('4');
+
+        self::assertSame(4, $this->repo->countActiveForOffering(7));
+    }
+
+    public function testHasActiveEnrollmentTrueWhenCountPositive(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/offering_id = %d AND student_id = %d AND status = %s/'), 7, 5, Enrollment::STATUS_ACTIVE)
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_var')->andReturn('1');
+
+        self::assertTrue($this->repo->hasActiveEnrollment(7, 5));
+    }
+
+    public function testHasActiveEnrollmentFalseWhenZero(): void
+    {
+        $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
+        $this->db->shouldReceive('get_var')->andReturn('0');
+
+        self::assertFalse($this->repo->hasActiveEnrollment(7, 5));
+    }
+
+    public function testFindAllActiveMapsRows(): void
+    {
+        $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
+        $this->db->shouldReceive('get_results')->andReturn([
+            (object) [
+                'id'            => '12',
+                'offering_id'   => '7',
+                'student_id'    => '5',
+                'instructor_id' => '3',
+                'status'        => Enrollment::STATUS_ACTIVE,
+                'payment_id'    => null,
+            ],
+        ]);
+
+        $all = $this->repo->findAllActive();
+
+        self::assertCount(1, $all);
+        self::assertInstanceOf(Enrollment::class, $all[0]);
+    }
+
+    public function testUpdateStatusRejectsInvalid(): void
+    {
+        self::assertFalse($this->repo->updateStatus(1, 'bogus'));
+    }
+
+    public function testUpdateStatusCallsWpdb(): void
+    {
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with('wp_us_group_enrollments', ['status' => Enrollment::STATUS_CANCELLED], ['id' => 1], ['%s'], ['%d'])
+            ->andReturn(1);
+
+        self::assertTrue($this->repo->updateStatus(1, Enrollment::STATUS_CANCELLED));
+    }
+}