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/Auth/RegistrationPage.php

@@ -0,0 +1,159 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+use Unsupervised\Schedular\Policy\AcceptanceRepository;
+use Unsupervised\Schedular\Policy\Policy;
+use Unsupervised\Schedular\Policy\PolicyAcceptance;
+use Unsupervised\Schedular\Policy\PolicyRepository;
+use Unsupervised\Schedular\Policy\PolicyVersionRepository;
+
+class RegistrationPage {
+
+	public function __construct(
+		private InviteRepository $invites,
+		private PolicyRepository $policies,
+		private PolicyVersionRepository $versions,
+		private AcceptanceRepository $acceptances,
+	) {}
+
+	/**
+	 * Renders the student registration shortcode output.
+	 *
+	 * @param array<string, string> $atts Shortcode attributes (unused — reserved for future options).
+	 */
+	public function render( array $atts ): string { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found
+		if ( is_user_logged_in() ) {
+			return '<p>' . esc_html__( 'You already have an account and are logged in.', 'unsupervised-schedular' ) . '</p>';
+		}
+
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- token identifies the invite; the form submit is nonce-checked below.
+		$token  = sanitize_text_field( wp_unslash( $_REQUEST['us_invite'] ?? '' ) );
+		$invite = '' !== $token ? $this->invites->findByToken( $token ) : null;
+
+		$error   = '';
+		$success = false;
+
+		if ( isset( $_POST['us_register'] ) && check_admin_referer( 'us_student_register' ) ) {
+			$result = $this->handleSubmit( $invite );
+			if ( true === $result ) {
+				$success = true;
+			} else {
+				$error = $result;
+			}
+		}
+
+		$policyForms = $this->signupPolicies();
+		$canRegister = null !== $invite && $invite->isPending();
+
+		ob_start();
+		include USC_PLUGIN_DIR . 'templates/frontend/register-page.php';
+		return (string) ob_get_clean();
+	}
+
+	/**
+	 * Process the submitted registration. Returns true on success or an error
+	 * message string on failure.
+	 */
+	private function handleSubmit( ?Invite $invite ): string|bool {
+		if ( null === $invite || ! $invite->isPending() ) {
+			return esc_html__( 'This invitation is invalid or has already been used.', 'unsupervised-schedular' );
+		}
+
+		// The submit nonce is verified by the caller (render) before this runs.
+		// phpcs:disable WordPress.Security.NonceVerification.Missing
+		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- passwords must not be sanitized.
+		$password    = (string) wp_unslash( $_POST['password'] ?? '' );
+		$displayName = sanitize_text_field( wp_unslash( $_POST['display_name'] ?? '' ) );
+
+		if ( strlen( $password ) < 8 ) {
+			return esc_html__( 'Please choose a password of at least 8 characters.', 'unsupervised-schedular' );
+		}
+
+		$policyForms = $this->signupPolicies();
+		$accepted    = array_map( 'absint', (array) ( $_POST['accept'] ?? [] ) );
+		// phpcs:enable WordPress.Security.NonceVerification.Missing
+
+		foreach ( $policyForms as $form ) {
+			if ( ! in_array( (int) $form['version']->id, $accepted, true ) ) {
+				return esc_html__( 'You must accept all required policies to register.', 'unsupervised-schedular' );
+			}
+		}
+
+		if ( email_exists( $invite->email ) ) {
+			return esc_html__( 'An account already exists for this email.', 'unsupervised-schedular' );
+		}
+
+		$userId = wp_insert_user(
+			[
+				'user_login'   => $invite->email,
+				'user_email'   => $invite->email,
+				'user_pass'    => $password,
+				'display_name' => '' !== $displayName ? $displayName : $invite->email,
+				'role'         => $invite->role,
+			]
+		);
+
+		if ( is_wp_error( $userId ) ) {
+			return esc_html__( 'Could not create the account. Please contact the studio.', 'unsupervised-schedular' );
+		}
+
+		$this->recordAcceptances( $policyForms, (int) $userId );
+		$this->invites->markAccepted( (int) $invite->id, (int) $userId );
+
+		wp_set_current_user( (int) $userId );
+		wp_set_auth_cookie( (int) $userId );
+
+		return true;
+	}
+
+	/**
+	 * Record account-time acceptances for each signup policy version.
+	 *
+	 * @param list<array{policy: Policy, version: \Unsupervised\Schedular\Policy\PolicyVersion}> $policyForms
+	 */
+	private function recordAcceptances( array $policyForms, int $userId ): void {
+		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.MissingUnslash -- IP is stored verbatim for audit.
+		$ip = sanitize_text_field( wp_unslash( $_SERVER['REMOTE_ADDR'] ?? '' ) );
+
+		foreach ( $policyForms as $form ) {
+			$this->acceptances->insert(
+				new PolicyAcceptance(
+					policyVersionId:  (int) $form['version']->id,
+					studentId:        $userId,
+					registrationType: PolicyAcceptance::REG_ACCOUNT,
+					registrationId:   $userId,
+					ipAddress:        '' !== $ip ? $ip : null,
+				)
+			);
+		}
+	}
+
+	/**
+	 * Signup-scoped policies that have a current published version.
+	 *
+	 * @return list<array{policy: Policy, version: \Unsupervised\Schedular\Policy\PolicyVersion}>
+	 */
+	private function signupPolicies(): array {
+		$out = [];
+
+		foreach ( $this->policies->findForScope( Policy::SCOPE_SIGNUP ) as $policy ) {
+			if ( null === $policy->currentVersionId ) {
+				continue;
+			}
+
+			$version = $this->versions->findById( $policy->currentVersionId );
+			if ( null === $version || ! $version->isPublished() ) {
+				continue;
+			}
+
+			$out[] = [
+				'policy'  => $policy,
+				'version' => $version,
+			];
+		}
+
+		return $out;
+	}
+}