Add account registration with signup policy acceptance

Implements #16: invite-only student self-registration through a front-end
page, accepting signup-scoped policies at account creation.

Policy domain:
- us_policies.acceptance_scope (signup/booking/both); Policy::appliesTo();
  PolicyRepository::findForScope(); scope threaded through PolicyService,
  the REST create, the admin controller, and the Policies form.
- PolicyAcceptance::REG_ACCOUNT (registration_id = the new user's ID).

Auth:
- Invite value object + InviteRepository; us_invites table.
- RegistrationController + Invites admin page (manage_students): invite an
  email, share the registration link, revoke.
- RegistrationPage ([us_student_register] shortcode): validates the invite
  token, collects name/password, renders signup-scoped published policies
  with required acceptance, creates the us_student user, records account-type
  acceptances, marks the invite accepted, and logs the user in.
- RoleManager: manage_students cap added to STUDIO_ADMIN_CAPS.

Invite-only is implemented; the us_registration_mode self_approval path is a
documented future seam.

Docs: docs/features/account-registration.md; policies.md updated.
Tests: tests/Unit/Auth/ (Invite, InviteRepository) plus Policy scope
updates. composer test (104), cs, and PHPStan level 6 all pass.

Refs #16

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/AdminMenu.php

@@ -5,6 +5,8 @@ namespace Unsupervised\Schedular;
 
 use Unsupervised\Schedular\Availability\AvailabilityController;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
+use Unsupervised\Schedular\Auth\InviteRepository;
+use Unsupervised\Schedular\Auth\RegistrationController;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Booking\LessonController;
@@ -24,13 +26,15 @@ class AdminMenu {
 	private OfferingController $offeringController;
 	private QuestionController $questionController;
 	private PolicyController $policyController;
+	private RegistrationController $registrationController;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites ) {
 		$this->availabilityController = new AvailabilityController( $availability, $offerings );
 		$this->lessonController       = new LessonController( $bookings );
 		$this->offeringController     = new OfferingController( $offerings );
 		$this->questionController     = new QuestionController( $questions, $offerings );
 		$this->policyController       = new PolicyController( $policies, $policyVersions, $policyService );
+		$this->registrationController = new RegistrationController( $invites );
 	}
 
 	public function register(): void {
@@ -92,6 +96,17 @@ class AdminMenu {
 			34
 		);
 
+		// Studio admin: invite students to register.
+		add_menu_page(
+			__( 'Invites', 'unsupervised-schedular' ),
+			__( 'Invites', 'unsupervised-schedular' ),
+			RoleManager::CAP_MANAGE_STUDENTS,
+			'us-invites',
+			[ $this->registrationController, 'renderPage' ],
+			'dashicons-email',
+			35
+		);
+
 		// Instructor: view their upcoming lessons.
 		add_menu_page(
 			__( 'My Lessons', 'unsupervised-schedular' ),