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/Auth/StudentController.php

@@ -8,6 +8,7 @@ use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Booking\Lesson;
 use Unsupervised\Schedular\GroupClass\Enrollment;
 use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
+use Unsupervised\Schedular\Guardian\GuardianService;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Payment\BillingMethodResolver;
 use Unsupervised\Schedular\Payment\Payment;
@@ -23,6 +24,7 @@ class StudentController {
 		private BillingMethodResolver $resolver,
 		private StudentHistory $history,
 		private StudentActions $actions,
+		private GuardianService $guardians,
 	) {}
 
 	public function renderPage(): void {
@@ -43,10 +45,14 @@ class StudentController {
 			fn( \WP_User $user ): array => [
 				'id'         => (int) $user->ID,
 				'name'       => $user->display_name,
-				'email'      => $user->user_email,
+				// A child's own address is an undeliverable placeholder, so the
+				// list shows the guardian's — the address an admin would use.
+				'email'      => $this->guardians->contactFor( (int) $user->ID )['email'],
 				'registered' => $user->user_registered,
 				'upcoming'   => $this->bookings->countUpcomingForStudent( (int) $user->ID ),
 				'enrolments' => $this->enrollments->countActiveForStudent( (int) $user->ID ),
+				'guardian'   => $this->guardians->guardianOf( (int) $user->ID ),
+				'children'   => $this->guardians->children( (int) $user->ID ),
 			],
 			array_filter(
 				get_users(
@@ -150,10 +156,19 @@ class StudentController {
 		$intake           = $this->history->intakeAnswers( (int) $student->ID );
 		$payments         = $canBilling ? $this->history->payments( (int) $student->ID ) : [];
 		$credits          = $canBilling ? $this->history->credits( (int) $student->ID ) : [];
-		$creditBalance    = $canBilling ? $this->history->creditBalance( (int) $student->ID ) : 0.0;
 		$creditCurrency   = $this->creditCurrency( $credits );
 
-		$backUrl = admin_url( 'admin.php?page=us-students' );
+		// The family panel, and the account whose balance actually settles this
+		// student's charges — a child's is their guardian's, so showing the
+		// child's own (always empty) balance would be actively misleading.
+		$guardian = $this->guardians->guardianOf( (int) $student->ID );
+		$children = $this->guardians->children( (int) $student->ID );
+		$payer    = $this->guardians->contactFor( (int) $student->ID );
+
+		$creditBalance = $canBilling ? $this->history->creditBalance( $payer['id'] ) : 0.0;
+
+		$backUrl  = admin_url( 'admin.php?page=us-students' );
+		$pageSlug = 'us-students';
 		include USC_PLUGIN_DIR . 'templates/admin/student-detail.php';
 	}