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/Auth/RegistrationPage.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Auth;
 
+use Unsupervised\Schedular\GroupClass\GroupAccessRepository;
 use Unsupervised\Schedular\Payment\StudioSettings;
 use Unsupervised\Schedular\Policy\AcceptanceRepository;
 use Unsupervised\Schedular\Policy\Policy;
@@ -38,6 +39,7 @@ class RegistrationPage {
 		private RegistrationMailer $mailer,
 		private QuestionRepository $questions,
 		private AnswerRepository $answers,
+		private GroupAccessRepository $access,
 	) {}
 
 	/**
@@ -203,6 +205,11 @@ class RegistrationPage {
 		if ( $inviteValid && ! $invite->isGroup() ) {
 			$this->invites->markAccepted( (int) $invite->id, (int) $userId );
 
+			// A personal invite may carry a group-class grant (invited by email);
+			// point any grants for this address at the new account so the class
+			// becomes enrollable for them.
+			$this->access->linkStudentByEmail( $email, (int) $userId );
+
 			wp_set_current_user( (int) $userId );
 			wp_set_auth_cookie( (int) $userId );