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/account-registration.md

@@ -165,3 +165,12 @@ No-op when no registration page is set.
 - `tests/Unit/Auth/RegistrationApprovalControllerTest.php`
 - `tests/Unit/Auth/RegistrationMailerTest.php`
 - `tests/Unit/Payment/StudioSettingsTest.php`
+
+## Parent/Guardian Signup
+The registration form also offers **"I'm registering as a parent or guardian"**,
+which reveals a repeatable child block (name, date of birth, and the
+account-scope questions asked **per child**). Each child becomes a login-less
+`us_student` user linked to the guardian, and the signup policies are recorded
+once per child with the guardian as the acceptor. Available on every signup path
+— personal invite, group link, and self-approval. See
+`parent-guardian-accounts.md`.