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/Policy/PolicyEndpoint.php

@@ -113,7 +113,12 @@ class PolicyEndpoint {
 			return new \WP_Error( 'duplicate_slug', __( 'A policy with that slug already exists.', 'unsupervised-schedular' ), [ 'status' => 409 ] );
 		}
 
-		$id = $this->service->createPolicy( $title, $slug );
+		$scope = (string) ( $request->get_param( 'acceptance_scope' ) ?? Policy::SCOPE_BOOKING );
+		if ( ! in_array( $scope, Policy::VALID_SCOPES, true ) ) {
+			return $this->invalid( __( 'Invalid acceptance scope.', 'unsupervised-schedular' ) );
+		}
+
+		$id = $this->service->createPolicy( $title, $slug, $scope );
 
 		return new \WP_REST_Response( [ 'id' => $id ], 201 );
 	}