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/Schema.php

@@ -140,6 +140,21 @@ class Schema {
                 KEY registration (registration_type, registration_id)
             ) {$charset};",
 
+			"CREATE TABLE {$prefix}us_group_enrollments (
+                id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                offering_id   BIGINT UNSIGNED NOT NULL,
+                student_id    BIGINT UNSIGNED NOT NULL,
+                instructor_id BIGINT UNSIGNED NOT NULL,
+                status        VARCHAR(20)     NOT NULL DEFAULT 'active',
+                payment_id    BIGINT UNSIGNED DEFAULT NULL,
+                enrolled_at   DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY offering_id (offering_id),
+                KEY student_id (student_id),
+                KEY instructor_id (instructor_id),
+                KEY status (status)
+            ) {$charset};",
+
 			"CREATE TABLE {$prefix}us_invites (
                 id               BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 email            VARCHAR(191)    NOT NULL,