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/policies.md

@@ -84,3 +84,9 @@ cover every policy's current version or the registration is rejected.
 - `tests/Unit/Policy/PolicyServiceTest.php`
 - `tests/Unit/Policy/PolicyControllerTest.php`
 - `tests/Unit/Policy/PolicyEndpointTest.php`
+
+## Who Accepted
+`us_policy_acceptances.accepted_by` records the person who actually ticked the
+box, where that differs from `student_id` — a guardian agreeing on a child's
+behalf. It defaults to 0, read back as "the student agreed for themselves"
+(`PolicyAcceptance::acceptorOrStudent()`). See `parent-guardian-accounts.md`.