Add open student registration with email confirmation and approval

Students could previously join by invite only. Add an optional
self-approval mode, toggled from Studio Settings → Registration: anyone
may sign up on the existing [us_student_register] page, confirm their
email via a tokenised link, and then be approved by a studio admin
before the account is usable.

- Enabling the toggle mirrors WordPress's own membership settings
  (users_can_register + default_role = us_student) and snapshots their
  previous values so disabling restores them.
- WordPress's native registration form is blocked while open
  registration is on (login_init redirect + registration_errors
  fail-safe + register_url) so it cannot bypass signup policy acceptance.
- Pending accounts: unconfirmed email cannot log in; confirmed but
  unapproved can log in but the booking capability is withheld and the
  booking page shows an "awaiting approval" screen.
- Approve/reject from Students → Pending Students; reject hard-deletes
  the account so the email is freed to re-apply.
- Invite registration is unchanged; both modes coexist.

Account lifecycle lives in user meta (RegistrationStatus); no new tables.

Closes #63

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Auth/EmailConfirmationHandler.php

@@ -0,0 +1,128 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+use Unsupervised\Schedular\Payment\StudioSettings;
+use Unsupervised\Schedular\Val;
+
+/**
+ * Handles the self-signup email-confirmation link, and keeps WordPress's own
+ * registration form from being used to bypass the studio's policy-accepting
+ * registration page while open registration is enabled.
+ */
+class EmailConfirmationHandler {
+
+	public function __construct(
+		private StudioSettings $settings,
+		private RegistrationMailer $mailer,
+	) {}
+
+	public function register(): void {
+		add_action( 'template_redirect', [ $this, 'maybeConfirm' ] );
+		add_filter( 'register_url', [ $this, 'registerUrl' ] );
+		// login_init fires at the top of wp-login.php for every request (GET form
+		// display AND a direct POST) before any registration processing, so it is
+		// the reliable choke point; registration_errors is a fail-safe in case a
+		// POST ever reaches register_new_user().
+		add_action( 'login_init', [ $this, 'blockNativeRegistration' ] );
+		add_filter( 'registration_errors', [ $this, 'blockRegistrationErrors' ], 10, 1 );
+	}
+
+	/**
+	 * Confirm a self-signup's email when the emailed `?us_confirm=<token>` link
+	 * is opened, then redirect back to the registration page with a result flag.
+	 */
+	public function maybeConfirm(): void {
+		if ( is_admin() ) {
+			return;
+		}
+
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- the token is itself the capability-bearing secret (like a password-reset key); nonces do not apply to an emailed link.
+		$rawToken = sanitize_text_field( Val::string( wp_unslash( $_GET['us_confirm'] ?? '' ) ) );
+		if ( '' === $rawToken ) {
+			return;
+		}
+
+		$base   = $this->registrationPageUrl();
+		$userId = RegistrationStatus::userIdForToken( $rawToken );
+
+		if ( null === $userId || RegistrationStatus::isTokenExpired( $userId, gmdate( 'Y-m-d H:i:s' ) ) ) {
+			wp_safe_redirect( add_query_arg( 'us_confirmed', 'expired', $base ) );
+			exit;
+		}
+
+		RegistrationStatus::confirmEmail( $userId );
+
+		$user = get_user_by( 'id', $userId );
+		if ( $user instanceof \WP_User ) {
+			$this->mailer->notifyAdminsPending( $user );
+		}
+
+		wp_safe_redirect( add_query_arg( 'us_confirmed', '1', $base ) );
+		exit;
+	}
+
+	/**
+	 * Point WordPress's own "Register" links at the studio registration page
+	 * while open registration is on and a page is configured.
+	 */
+	public function registerUrl( string $url ): string {
+		if ( ! $this->settings->openRegistrationEnabled() ) {
+			return $url;
+		}
+
+		$pageId = Val::int( get_option( RegistrationController::OPTION_PAGE, 0 ) );
+
+		return $pageId > 0 ? (string) get_permalink( $pageId ) : $url;
+	}
+
+	/**
+	 * Redirect any `wp-login.php?action=register` request (GET or POST) to the
+	 * studio registration page, so the bare native form — which cannot collect
+	 * required policy acceptances — is never used.
+	 */
+	public function blockNativeRegistration(): void {
+		if ( ! $this->settings->openRegistrationEnabled() ) {
+			return;
+		}
+
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- read-only routing decision; no state is changed here.
+		$action = sanitize_key( Val::string( wp_unslash( $_REQUEST['action'] ?? '' ) ) );
+		if ( 'register' !== $action ) {
+			return;
+		}
+
+		$pageId = Val::int( get_option( RegistrationController::OPTION_PAGE, 0 ) );
+		if ( $pageId <= 0 ) {
+			return;
+		}
+
+		wp_safe_redirect( (string) get_permalink( $pageId ) );
+		exit;
+	}
+
+	/**
+	 * Fail-safe: reject any native registration attempt while open registration
+	 * is on, so `register_new_user()` can never create a policy-less account.
+	 *
+	 * @param \WP_Error $errors Accumulated registration errors.
+	 * @return \WP_Error
+	 */
+	public function blockRegistrationErrors( \WP_Error $errors ): \WP_Error {
+		if ( $this->settings->openRegistrationEnabled() ) {
+			$errors->add(
+				'us_registration_redirect',
+				esc_html__( 'Please register on the studio registration page.', 'unsupervised-schedular' )
+			);
+		}
+
+		return $errors;
+	}
+
+	private function registrationPageUrl(): string {
+		$pageId = Val::int( get_option( RegistrationController::OPTION_PAGE, 0 ) );
+
+		return $pageId > 0 ? (string) get_permalink( $pageId ) : home_url( '/' );
+	}
+}