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/Plugin.php

@@ -3,10 +3,12 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular;
 
+use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Policy\AcceptanceRepository;
 use Unsupervised\Schedular\Policy\PolicyRepository;
 use Unsupervised\Schedular\Policy\PolicyService;
 use Unsupervised\Schedular\Policy\PolicyVersionRepository;
@@ -25,10 +27,12 @@ class Plugin {
 		$policies       = new PolicyRepository( $wpdb );
 		$policyVersions = new PolicyVersionRepository( $wpdb );
 		$policyService  = new PolicyService( $policies, $policyVersions );
+		$acceptances    = new AcceptanceRepository( $wpdb );
+		$invites        = new InviteRepository( $wpdb );
 
 		( new RoleManager() )->register();
-		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService ) )->register();
+		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites ) )->register();
 		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService ) )->register();
-		( new ShortcodeRegistrar() )->register();
+		( new ShortcodeRegistrar( $invites, $policies, $policyVersions, $acceptances ) )->register();
 	}
 }