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/Payment/Credit.php

@@ -26,6 +26,12 @@ class Credit {
 		public readonly int $studentId,
 		public readonly float $amount,
 		public readonly float $remaining,
+		/**
+		 * The account holding this balance — a child's guardian, or 0 meaning
+		 * "the student themselves". A family's credits all sit on the guardian,
+		 * so one child's cancellation can settle a sibling's charge.
+		 */
+		public readonly int $payerId = 0,
 		public readonly string $currency = 'CAD',
 		public readonly ?int $sourcePaymentId = null,
 		public readonly ?int $sourceLessonId = null,
@@ -41,6 +47,7 @@ class Credit {
 			studentId:       Val::int( $row->student_id ),
 			amount:          Val::float( $row->amount ),
 			remaining:       Val::float( $row->remaining ),
+			payerId:         Val::int( $row->payer_id ?? 0 ),
 			currency:        Val::string( $row->currency ),
 			sourcePaymentId: Val::intOrNull( $row->source_payment_id ?? null ),
 			sourceLessonId:  Val::intOrNull( $row->source_lesson_id ?? null ),
@@ -56,6 +63,15 @@ class Credit {
 		return self::STATUS_AVAILABLE === $this->status && $this->remaining > 0.0;
 	}
 
+	/**
+	 * Whose balance this credit sits in: the recorded payer, falling back to the
+	 * student. Callers go through here rather than reading `payerId`, so the `0`
+	 * default of a pre-guardian credit never leaks out as a user id.
+	 */
+	public function payerOrStudent(): int {
+		return $this->payerId > 0 ? $this->payerId : $this->studentId;
+	}
+
 	/**
 	 * Returns a plain array representation of the credit.
 	 *
@@ -65,6 +81,7 @@ class Credit {
 		return [
 			'id'                => $this->id,
 			'student_id'        => $this->studentId,
+			'payer_id'          => $this->payerOrStudent(),
 			'amount'            => $this->amount,
 			'remaining'         => $this->remaining,
 			'currency'          => $this->currency,