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]>

src/ShortcodeRegistrar.php

@@ -7,6 +7,7 @@ use Unsupervised\Schedular\Auth\LoginPage;
 use Unsupervised\Schedular\Auth\RegistrationPage;
 use Unsupervised\Schedular\Booking\BookingPage;
 use Unsupervised\Schedular\GroupClass\GroupClassPage;
+use Unsupervised\Schedular\Guardian\FamilyPage;
 use Unsupervised\Schedular\Payment\StudioSettings;
 
 class ShortcodeRegistrar {
@@ -16,6 +17,7 @@ class ShortcodeRegistrar {
 		private LoginPage $loginPage,
 		private RegistrationPage $registrationPage,
 		private GroupClassPage $groupClassPage,
+		private FamilyPage $familyPage,
 	) {}
 
 	public function register(): void {
@@ -23,10 +25,14 @@ class ShortcodeRegistrar {
 		add_shortcode( 'us_student_login', self::shortcode( [ $this->loginPage, 'render' ] ) );
 		add_shortcode( 'us_student_register', self::shortcode( [ $this->registrationPage, 'render' ] ) );
 		add_shortcode( 'us_group_classes', self::shortcode( [ $this->groupClassPage, 'render' ] ) );
+		add_shortcode( 'us_family', self::shortcode( [ $this->familyPage, 'render' ] ) );
 		// Process registration submissions before output so the invite branch's
 		// auth cookie is actually sent (render() runs too late, during the_content).
 		add_action( 'template_redirect', [ $this->registrationPage, 'maybeHandleSubmit' ] );
 		add_action( 'template_redirect', [ $this->registrationPage, 'maybeRedirectToRegistrationPage' ] );
+		// Same reason as registration: the family form redirects after handling,
+		// which render() (running during the_content) is too late to do.
+		add_action( 'template_redirect', [ $this->familyPage, 'maybeHandleSubmit' ] );
 		add_action( 'wp_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 	}
 
@@ -76,8 +82,11 @@ class ShortcodeRegistrar {
 		// Price formatting and the pay agreement, shared by booking and enrolment.
 		wp_register_script( 'us-scheduler-pricing', USC_PLUGIN_URL . 'assets/js/pricing.js', [ 'us-scheduler-payment' ], USC_VERSION, true );
 
-		wp_register_script( 'us-scheduler', USC_PLUGIN_URL . 'assets/js/booking.js', [ 'us-scheduler-pricing' ], USC_VERSION, true );
-		wp_register_script( 'us-scheduler-group', USC_PLUGIN_URL . 'assets/js/group-classes.js', [ 'us-scheduler-pricing' ], USC_VERSION, true );
+		// The "who is this for?" picker, shared by booking and enrolment.
+		wp_register_script( 'us-scheduler-guardian', USC_PLUGIN_URL . 'assets/js/guardian.js', [], USC_VERSION, true );
+
+		wp_register_script( 'us-scheduler', USC_PLUGIN_URL . 'assets/js/booking.js', [ 'us-scheduler-pricing', 'us-scheduler-guardian' ], USC_VERSION, true );
+		wp_register_script( 'us-scheduler-group', USC_PLUGIN_URL . 'assets/js/group-classes.js', [ 'us-scheduler-pricing', 'us-scheduler-guardian' ], USC_VERSION, true );
 
 		// Progressive enhancement for the two-step registration form (no dependencies).
 		wp_register_script( 'us-scheduler-register', USC_PLUGIN_URL . 'assets/js/register.js', [], USC_VERSION, true );