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/Policy/AcceptanceRepositoryTest.php

@@ -36,13 +36,15 @@ class AcceptanceRepositoryTest extends TestCase
                         && $d['student_id'] === 5
                         && $d['registration_type'] === PolicyAcceptance::REG_LESSON
                         && $d['registration_id'] === 12
+                        // No explicit acceptor: the student agreed for themselves.
+                        && $d['accepted_by'] === 5
                         && $d['ip_address'] === '203.0.113.7';
                 }),
-                ['%d', '%d', '%s', '%d', '%s', '%s']
+                ['%d', '%d', '%d', '%s', '%d', '%s', '%s']
             );
         $this->db->insert_id = 1;
 
-        $acceptance = new PolicyAcceptance(9, 5, PolicyAcceptance::REG_LESSON, 12, '203.0.113.7');
+        $acceptance = new PolicyAcceptance(9, 5, PolicyAcceptance::REG_LESSON, 12, ipAddress: '203.0.113.7');
 
         self::assertSame(1, $this->repo->insert($acceptance));
     }