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]>

assets/js/group-classes.js

@@ -17,6 +17,10 @@
     // enrolment controls.
     const singleOfferingId = Number(app.dataset.offering || 0);
 
+    // Who this account may enrol — children first, the account holder last, so a
+    // guardian's default selection is a child. One entry means no picker.
+    const students = window.usGuardian.parseStudents(app.dataset.students);
+
     function apiFetch(path, options = {}) {
         return fetch(restUrl + path, {
             ...options,
@@ -202,6 +206,7 @@
             <div class="us-register">
                 <h3>${escHtml(offering.title)}</h3>
                 <form id="us-enrol-form">
+                    ${window.usGuardian.selectorHtml(students, 'us-enrol-student')}
                     ${questions.map(questionField).join('')}
                     ${policies.map(policyField).join('')}
                     ${window.usPricing.summaryHtml(offering)}
@@ -239,6 +244,7 @@
             method: 'POST',
             body: JSON.stringify({
                 offering_id: offering.id,
+                student_id: window.usGuardian.selectedId('us-enrol-student'),
                 answers,
                 accepted_policy_version_ids: accepted,
             }),