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/Schema.php

@@ -100,9 +100,11 @@ class Schema {
                 title              VARCHAR(191)    NOT NULL,
                 slug               VARCHAR(191)    NOT NULL,
                 current_version_id BIGINT UNSIGNED DEFAULT NULL,
+                acceptance_scope   VARCHAR(20)     NOT NULL DEFAULT 'booking',
                 created_at         DATETIME        NOT NULL,
                 PRIMARY KEY (id),
-                UNIQUE KEY slug (slug)
+                UNIQUE KEY slug (slug),
+                KEY acceptance_scope (acceptance_scope)
             ) {$charset};",
 
 			"CREATE TABLE {$prefix}us_policy_versions (
@@ -131,6 +133,22 @@ class Schema {
                 KEY student_id (student_id),
                 KEY registration (registration_type, registration_id)
             ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_invites (
+                id               BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                email            VARCHAR(191)    NOT NULL,
+                token            VARCHAR(64)     NOT NULL,
+                role             VARCHAR(32)     NOT NULL DEFAULT 'us_student',
+                status           VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                invited_by       BIGINT UNSIGNED DEFAULT NULL,
+                accepted_user_id BIGINT UNSIGNED DEFAULT NULL,
+                created_at       DATETIME        NOT NULL,
+                accepted_at      DATETIME        DEFAULT NULL,
+                PRIMARY KEY (id),
+                UNIQUE KEY token (token),
+                KEY email (email),
+                KEY status (status)
+            ) {$charset};",
 		];
 	}
 }