Add invite-only group classes

Group classes can now be marked invite-only (us_offerings.access_mode).
Invite-only classes are hidden from the public catalog and reachable only
when the instructor lets someone in via one of three paths, managed from
My Lessons -> My Group Classes:

- Add students directly: enrols them now with a pending payment.
- Make available: grants registered students access to self-enrol through
  the normal paid flow (multi-select, emailed a notice).
- Invite by email: tokenised registration invite tied to the class for a
  non-account address; after they register the class becomes enrollable.
  Reuses an existing pending invite instead of sending a second link.

New us_group_access table records grants; GET /offerings merges granted
invite-only classes for the caller; enrolment requires a grant
(403 invite_required) and flips it to enrolled on success.

composer test (487), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/GroupClass/EnrollmentEndpoint.php

@@ -19,6 +19,7 @@ class EnrollmentEndpoint {
 		private OfferingRepository $offerings,
 		private RegistrationGate $gate,
 		private PaymentService $payments,
+		private GroupAccessRepository $access,
 	) {}
 
 	/**
@@ -88,6 +89,12 @@ class EnrollmentEndpoint {
 			return new \WP_Error( 'already_enrolled', __( 'You are already enrolled in this class.', 'unsupervised-schedular' ), [ 'status' => 409 ] );
 		}
 
+		// Invite-only classes can only be enrolled in by students who were granted
+		// access (or added directly); everyone else never sees the class at all.
+		if ( $offering->isInviteOnly() && ! $this->access->hasGrant( $offeringId, $studentId ) ) {
+			return new \WP_Error( 'invite_required', __( 'This class is by invitation only.', 'unsupervised-schedular' ), [ 'status' => 403 ] );
+		}
+
 		if ( null !== $offering->capacity && $this->enrollments->countActiveForOffering( $offeringId ) >= $offering->capacity ) {
 			return new \WP_Error( 'class_full', __( 'This class is full.', 'unsupervised-schedular' ), [ 'status' => 409 ] );
 		}
@@ -110,6 +117,12 @@ class EnrollmentEndpoint {
 
 		$this->gate->record( PolicyAcceptance::REG_ENROLLMENT, $id, $studentId, $offeringId, $answers, $acceptedVersionIds, $this->clientIp() );
 
+		// Mark the access grant used so instructor rosters distinguish invited
+		// students from enrolled ones (a no-op for public classes).
+		if ( $offering->isInviteOnly() ) {
+			$this->access->markEnrolled( $offeringId, $studentId );
+		}
+
 		$payment = null;
 		if ( $offering->price > 0.0 ) {
 			$payment = $this->payments->createForRegistration( Payment::REG_ENROLLMENT, $id, $studentId, $offering->instructorId, $offering->price, $offering->currency, $offering->etransferEmail );