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]>

tests/Unit/Payment/StripeGatewayTest.php

@@ -29,7 +29,7 @@ class StripeGatewayTest extends TestCase
 
     private function payment(): Payment
     {
-        return new Payment(5, 3, Payment::REG_LESSON, 12, 35.00, 'CAD', Payment::METHOD_CARD, Payment::STATUS_PENDING, id: 90);
+        return new Payment(5, 3, Payment::REG_LESSON, 12, 35.00, currency: 'CAD', method: Payment::METHOD_CARD, status: Payment::STATUS_PENDING, id: 90);
     }
 
     public function testCreateIntentReturnsNullWhenNotConfigured(): void