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/Auth/RoleManager.php

@@ -14,6 +14,7 @@ class RoleManager {
 	public const CAP_BOOK_LESSON         = 'book_lesson';
 
 	public const CAP_MANAGE_INSTRUCTORS = 'manage_instructors';
+	public const CAP_MANAGE_STUDENTS    = 'manage_students';
 	public const CAP_MANAGE_OFFERINGS   = 'manage_offerings';
 	public const CAP_MANAGE_QUESTIONS   = 'manage_questions';
 	public const CAP_MANAGE_POLICIES    = 'manage_policies';
@@ -31,6 +32,7 @@ class RoleManager {
 	 */
 	public const STUDIO_ADMIN_CAPS = [
 		self::CAP_MANAGE_INSTRUCTORS,
+		self::CAP_MANAGE_STUDENTS,
 		self::CAP_MANAGE_OFFERINGS,
 		self::CAP_MANAGE_QUESTIONS,
 		self::CAP_MANAGE_POLICIES,