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/ShortcodeRegistrar.php

@@ -3,22 +3,35 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular;
 
+use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\LoginPage;
+use Unsupervised\Schedular\Auth\RegistrationPage;
 use Unsupervised\Schedular\Booking\BookingPage;
+use Unsupervised\Schedular\Policy\AcceptanceRepository;
+use Unsupervised\Schedular\Policy\PolicyRepository;
+use Unsupervised\Schedular\Policy\PolicyVersionRepository;
 
 class ShortcodeRegistrar {
 
 	private BookingPage $bookingPage;
 	private LoginPage $loginPage;
+	private RegistrationPage $registrationPage;
 
-	public function __construct() {
-		$this->bookingPage = new BookingPage();
-		$this->loginPage   = new LoginPage();
+	public function __construct(
+		InviteRepository $invites,
+		PolicyRepository $policies,
+		PolicyVersionRepository $policyVersions,
+		AcceptanceRepository $acceptances,
+	) {
+		$this->bookingPage      = new BookingPage();
+		$this->loginPage        = new LoginPage();
+		$this->registrationPage = new RegistrationPage( $invites, $policies, $policyVersions, $acceptances );
 	}
 
 	public function register(): void {
 		add_shortcode( 'us_booking', [ $this->bookingPage, 'render' ] );
 		add_shortcode( 'us_student_login', [ $this->loginPage, 'render' ] );
+		add_shortcode( 'us_student_register', [ $this->registrationPage, 'render' ] );
 		add_action( 'wp_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 	}