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>

templates/admin/policies.php

@@ -1,6 +1,7 @@
 <?php
 declare(strict_types=1);
 
+use Unsupervised\Schedular\Policy\Policy;
 use Unsupervised\Schedular\Policy\PolicyVersion;
 
 if (! defined('ABSPATH')) {
@@ -31,6 +32,16 @@ if (! defined('ABSPATH')) {
                     <input type="text" name="slug" id="slug" class="regular-text" placeholder="<?php esc_attr_e('e.g. cancellation (defaults from title)', 'unsupervised-schedular'); ?>">
                 </td>
             </tr>
+            <tr>
+                <th><label for="acceptance_scope"><?php esc_html_e('Accept at', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <select name="acceptance_scope" id="acceptance_scope">
+                        <option value="<?php echo esc_attr(Policy::SCOPE_BOOKING); ?>" selected><?php esc_html_e('Booking', 'unsupervised-schedular'); ?></option>
+                        <option value="<?php echo esc_attr(Policy::SCOPE_SIGNUP); ?>"><?php esc_html_e('Account signup', 'unsupervised-schedular'); ?></option>
+                        <option value="<?php echo esc_attr(Policy::SCOPE_BOTH); ?>"><?php esc_html_e('Both', 'unsupervised-schedular'); ?></option>
+                    </select>
+                </td>
+            </tr>
         </table>
         <?php submit_button(esc_html__('Add Policy', 'unsupervised-schedular')); ?>
     </form>