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/frontend/register-page.php

@@ -0,0 +1,67 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+/**
+ * @var \Unsupervised\Schedular\Auth\Invite|null $invite
+ * @var bool   $canRegister
+ * @var bool   $success
+ * @var string $error
+ * @var list<array{policy: \Unsupervised\Schedular\Policy\Policy, version: \Unsupervised\Schedular\Policy\PolicyVersion}> $policyForms
+ */
+?>
+<div class="us-register-form">
+    <?php if ($success) : ?>
+        <p class="us-success"><?php esc_html_e('Your account has been created and you are now logged in.', 'unsupervised-schedular'); ?></p>
+    <?php elseif (! $canRegister) : ?>
+        <p><?php esc_html_e('Registration is by invitation only. Please use the link from your invitation email, or contact the studio.', 'unsupervised-schedular'); ?></p>
+    <?php else : ?>
+        <?php if ($error !== '') : ?>
+            <p class="us-error" role="alert"><?php echo esc_html($error); ?></p>
+        <?php endif; ?>
+
+        <form method="post" action="">
+            <?php wp_nonce_field('us_student_register'); ?>
+            <input type="hidden" name="us_invite" value="<?php echo esc_attr($invite->token); ?>">
+
+            <p>
+                <label for="us-reg-email"><?php esc_html_e('Email', 'unsupervised-schedular'); ?></label>
+                <input type="email" id="us-reg-email" value="<?php echo esc_attr($invite->email); ?>" readonly>
+            </p>
+            <p>
+                <label for="us-reg-name"><?php esc_html_e('Your name', 'unsupervised-schedular'); ?></label>
+                <input type="text" name="display_name" id="us-reg-name" autocomplete="name" required>
+            </p>
+            <p>
+                <label for="us-reg-pass"><?php esc_html_e('Password', 'unsupervised-schedular'); ?></label>
+                <input type="password" name="password" id="us-reg-pass" autocomplete="new-password" minlength="8" required>
+            </p>
+
+            <?php if (! empty($policyForms)) : ?>
+                <fieldset class="us-policies">
+                    <legend><?php esc_html_e('Policies', 'unsupervised-schedular'); ?></legend>
+                    <?php foreach ($policyForms as $form) : ?>
+                        <div class="us-policy">
+                            <h4><?php echo esc_html($form['policy']->title); ?></h4>
+                            <div class="us-policy-body"><?php echo wp_kses_post((string) $form['version']->body); ?></div>
+                            <label>
+                                <input type="checkbox" name="accept[]" value="<?php echo esc_attr((string) $form['version']->id); ?>" required>
+                                <?php
+                                /* translators: %s: policy title */
+                                echo esc_html(sprintf(__('I have read and agree to the %s.', 'unsupervised-schedular'), $form['policy']->title));
+                                ?>
+                            </label>
+                        </div>
+                    <?php endforeach; ?>
+                </fieldset>
+            <?php endif; ?>
+
+            <p>
+                <input type="submit" name="us_register" value="<?php esc_attr_e('Create Account', 'unsupervised-schedular'); ?>">
+            </p>
+        </form>
+    <?php endif; ?>
+</div>