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/Guardian/GuardianLink.php

@@ -0,0 +1,47 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Guardian;
+
+use Unsupervised\Schedular\Val;
+
+/**
+ * One parent/guardian ↔ child link. The child is a real (login-less) WordPress
+ * user, so `studentId` is a `wp_users` ID exactly like every other student id in
+ * the schema — this row only records who books and pays on their behalf.
+ */
+class GuardianLink {
+
+	public function __construct(
+		public readonly int $guardianId,
+		public readonly int $studentId,
+		public readonly string $relationship = '',
+		public readonly ?string $createdAt = null,
+		public readonly ?int $id = null,
+	) {}
+
+	public static function fromRow( \stdClass $row ): self {
+		return new self(
+			guardianId:   Val::int( $row->guardian_id ),
+			studentId:    Val::int( $row->student_id ),
+			relationship: Val::string( $row->relationship ?? '' ),
+			createdAt:    Val::stringOrNull( $row->created_at ?? null ),
+			id:           Val::int( $row->id ),
+		);
+	}
+
+	/**
+	 * Returns a plain array representation of the link.
+	 *
+	 * @return array<string, mixed>
+	 */
+	public function toArray(): array {
+		return [
+			'id'           => $this->id,
+			'guardian_id'  => $this->guardianId,
+			'student_id'   => $this->studentId,
+			'relationship' => $this->relationship,
+			'created_at'   => $this->createdAt,
+		];
+	}
+}