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/invites.php

@@ -0,0 +1,64 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+/** @var list<\Unsupervised\Schedular\Auth\Invite> $pendingInvites */
+?>
+<div class="wrap">
+    <h1><?php esc_html_e('Invites', 'unsupervised-schedular'); ?></h1>
+    <p class="description"><?php esc_html_e('Invite a student by email, then send them the registration link below. They complete signup and accept any required policies through the [us_student_register] page.', 'unsupervised-schedular'); ?></p>
+
+    <h2><?php esc_html_e('Invite a Student', 'unsupervised-schedular'); ?></h2>
+    <form method="post">
+        <?php wp_nonce_field('usc_invite_action'); ?>
+        <input type="hidden" name="usc_action" value="invite">
+        <table class="form-table">
+            <tr>
+                <th><label for="email"><?php esc_html_e('Email', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="email" name="email" id="email" class="regular-text" required></td>
+            </tr>
+        </table>
+        <?php submit_button(esc_html__('Send Invite', 'unsupervised-schedular')); ?>
+    </form>
+
+    <h2><?php esc_html_e('Pending Invites', 'unsupervised-schedular'); ?></h2>
+
+    <?php if (empty($pendingInvites)) : ?>
+        <p><?php esc_html_e('No pending invites.', 'unsupervised-schedular'); ?></p>
+    <?php else : ?>
+        <table class="wp-list-table widefat fixed striped">
+            <thead>
+                <tr>
+                    <th><?php esc_html_e('Email', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Registration link', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Actions', 'unsupervised-schedular'); ?></th>
+                </tr>
+            </thead>
+            <tbody>
+                <?php foreach ($pendingInvites as $invite) : ?>
+                    <?php $link = esc_url(add_query_arg('us_invite', $invite->token, home_url('/'))); ?>
+                    <tr>
+                        <td><?php echo esc_html($invite->email); ?></td>
+                        <td>
+                            <input type="text" class="large-text code" readonly value="<?php echo esc_attr($link); ?>" onclick="this.select()">
+                            <span class="description"><?php esc_html_e('Point this token (?us_invite=…) at the page containing the registration shortcode.', 'unsupervised-schedular'); ?></span>
+                        </td>
+                        <td>
+                            <form method="post" style="display:inline;">
+                                <?php wp_nonce_field('usc_invite_action'); ?>
+                                <input type="hidden" name="usc_action" value="revoke">
+                                <input type="hidden" name="invite_id" value="<?php echo esc_attr((string) $invite->id); ?>">
+                                <button type="submit" class="button button-small button-link-delete">
+                                    <?php esc_html_e('Revoke', 'unsupervised-schedular'); ?>
+                                </button>
+                            </form>
+                        </td>
+                    </tr>
+                <?php endforeach; ?>
+            </tbody>
+        </table>
+    <?php endif; ?>
+</div>