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]>

docs/features/registration-questions.md

@@ -89,3 +89,12 @@ through the server-rendered admin page and read directly by `RegistrationPage`.
 - `tests/Unit/Registration/AnswerTest.php`
 - `tests/Unit/Auth/RegistrationPageTest.php`
 - `tests/Unit/Auth/StudentHistoryTest.php`
+
+## Per-Child Answers
+For a parent/guardian signup, **account-scope** questions are asked **once per
+child** rather than once per guardian — in practice they describe the student
+(instrument, level, school), not the account holder. Each answer's `student_id`
+and `registration_id` are the child's user ID, so a studio admin reading a
+child's screen sees the information that describes them. The guardian's family
+screen asks the same questions when a child is added later. See
+`parent-guardian-accounts.md`.