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/Installer.php

@@ -5,7 +5,10 @@ namespace Unsupervised\Schedular;
 
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
+use Unsupervised\Schedular\Payment\CreditRepository;
+use Unsupervised\Schedular\Payment\PaymentRepository;
 use Unsupervised\Schedular\Payment\ScheduledBillingRunner;
+use Unsupervised\Schedular\Policy\AcceptanceRepository;
 
 class Installer {
 
@@ -50,5 +53,13 @@ class Installer {
 		}
 
 		( new AvailabilityRepository( $wpdb ) )->splitOversizedWindows();
+
+		// Guardian accounts introduced "who pays" / "who agreed" alongside "who the
+		// student is". Every row written before then had them one and the same, so
+		// point the new columns at the student rather than leaving them 0 — the
+		// balance and acceptance lookups key on them directly.
+		( new PaymentRepository( $wpdb ) )->backfillPayerIds();
+		( new CreditRepository( $wpdb ) )->backfillPayerIds();
+		( new AcceptanceRepository( $wpdb ) )->backfillAcceptedBy();
 	}
 }