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 <[email protected]>

src/GroupClass/EnrollmentRepository.php

@@ -0,0 +1,129 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\GroupClass;
+
+class EnrollmentRepository {
+
+	private string $table;
+
+	public function __construct( private \wpdb $db ) {
+		$this->table = $db->prefix . 'us_group_enrollments';
+	}
+
+	public function insert( Enrollment $enrollment ): int {
+		$this->db->insert(
+			$this->table,
+			[
+				'offering_id'   => $enrollment->offeringId,
+				'student_id'    => $enrollment->studentId,
+				'instructor_id' => $enrollment->instructorId,
+				'status'        => $enrollment->status,
+				'payment_id'    => $enrollment->paymentId,
+				'enrolled_at'   => current_time( 'mysql' ),
+			],
+			[ '%d', '%d', '%d', '%s', '%d', '%s' ]
+		);
+
+		return $this->db->insert_id;
+	}
+
+	public function findById( int $id ): ?Enrollment {
+		$row = $this->db->get_row(
+			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )
+		);
+
+		return $row ? Enrollment::fromRow( $row ) : null;
+	}
+
+	/**
+	 * Count active enrolments for an offering (capacity check).
+	 */
+	public function countActiveForOffering( int $offeringId ): int {
+		return (int) $this->db->get_var(
+			$this->db->prepare(
+				"SELECT COUNT(*) FROM {$this->table} WHERE offering_id = %d AND status = %s",
+				$offeringId,
+				Enrollment::STATUS_ACTIVE
+			)
+		);
+	}
+
+	/**
+	 * Whether a student already holds an active enrolment in an offering.
+	 */
+	public function hasActiveEnrollment( int $offeringId, int $studentId ): bool {
+		$count = (int) $this->db->get_var(
+			$this->db->prepare(
+				"SELECT COUNT(*) FROM {$this->table} WHERE offering_id = %d AND student_id = %d AND status = %s",
+				$offeringId,
+				$studentId,
+				Enrollment::STATUS_ACTIVE
+			)
+		);
+
+		return $count > 0;
+	}
+
+	/**
+	 * A student's enrolments, newest first.
+	 *
+	 * @return list<Enrollment>
+	 */
+	public function findByStudent( int $studentId ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE student_id = %d ORDER BY enrolled_at DESC",
+				$studentId
+			)
+		);
+
+		return array_map( Enrollment::fromRow( ... ), $rows ?? [] );
+	}
+
+	/**
+	 * Enrolments in an instructor's group classes, newest first.
+	 *
+	 * @return list<Enrollment>
+	 */
+	public function findByInstructor( int $instructorId ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE instructor_id = %d ORDER BY enrolled_at DESC",
+				$instructorId
+			)
+		);
+
+		return array_map( Enrollment::fromRow( ... ), $rows ?? [] );
+	}
+
+	/**
+	 * All active enrolments across instructors (studio-admin view).
+	 *
+	 * @return list<Enrollment>
+	 */
+	public function findAllActive(): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE status = %s ORDER BY enrolled_at DESC",
+				Enrollment::STATUS_ACTIVE
+			)
+		);
+
+		return array_map( Enrollment::fromRow( ... ), $rows ?? [] );
+	}
+
+	public function updateStatus( int $id, string $status ): bool {
+		if ( ! in_array( $status, Enrollment::VALID_STATUSES, true ) ) {
+			return false;
+		}
+
+		return (bool) $this->db->update(
+			$this->table,
+			[ 'status' => $status ],
+			[ 'id' => $id ],
+			[ '%s' ],
+			[ '%d' ]
+		);
+	}
+}