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 <[email protected]>

src/Policy/PolicyController.php

@@ -40,9 +40,14 @@ class PolicyController {
 			$title   = sanitize_text_field( wp_unslash( $_POST['title'] ?? '' ) );
 			$slugRaw = sanitize_text_field( wp_unslash( $_POST['slug'] ?? '' ) );
 			$slug    = sanitize_title( '' !== $slugRaw ? $slugRaw : $title );
+			$scope   = sanitize_key( wp_unslash( $_POST['acceptance_scope'] ?? Policy::SCOPE_BOOKING ) );
+
+			if ( ! in_array( $scope, Policy::VALID_SCOPES, true ) ) {
+				$scope = Policy::SCOPE_BOOKING;
+			}
 
 			if ( '' !== $title && '' !== $slug && null === $this->policies->findBySlug( $slug ) ) {
-				$this->service->createPolicy( $title, $slug );
+				$this->service->createPolicy( $title, $slug, $scope );
 			}
 
 			return;