Let parents register once and book for their children

A parent registers once and manages lessons for one or more children, who
need no login of their own. A child is a real wp_users row with the student
role but no usable login — so student_id keeps meaning "a WordPress user"
on every table, and booking, credits, policies and enrolments work unchanged.
A us_guardians link table maps guardian to child.

The signup form gains a parent/guardian tick that reveals a block per child,
with the account-signup questions asked per child rather than per guardian
— they describe the student, not the account holder. Signup policies are
recorded once per child with the guardian as the acceptor, which is the
record that actually means something. A family that half-creates is rolled
back entirely rather than leaving a guardian who cannot re-register.

The booking and enrolment forms gain a "Who is this for?" picker listing
children first, so the default selection is never the parent — booking for
the wrong child is correctable, quietly billing a parent for their kid's
lesson is not. POST /bookings and POST /enrollments take an optional
student_id honoured only for that child's guardian; anything else is a 403.
That check is the authorisation boundary of the feature.

Payments and credits gain a payer: the charge names the child it was for and
the guardian who owes it, so per-child reporting is unchanged while notices,
receipts and the payment step reach the parent. Credit is held by the payer,
so one child's cancellation can settle a sibling's charge, and the daily
billing scan sends a guardian one notice covering every child.

Closes #132

Co-Authored-By: Claude Opus 5 <[email protected]>

src/GroupClass/GroupClassPage.php

@@ -4,10 +4,13 @@ declare(strict_types=1);
 namespace Unsupervised\Schedular\GroupClass;
 
 use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Guardian\GuardianService;
 use Unsupervised\Schedular\Val;
 
 class GroupClassPage {
 
+	public function __construct( private GuardianService $guardians ) {}
+
 	/**
 	 * Renders the group-class enrolment shortcode output.
 	 *
@@ -39,6 +42,10 @@ class GroupClassPage {
 
 		$offeringId = absint( Val::int( $atts['offering'] ?? $atts['offeringId'] ?? 0 ) );
 
+		// Who this account may enrol — children first, the account holder last, so
+		// a guardian's default choice is a child rather than themselves.
+		$students = $this->guardians->bookableStudents( get_current_user_id() );
+
 		ob_start();
 		include USC_PLUGIN_DIR . 'templates/frontend/group-classes-page.php';
 		return (string) ob_get_clean();