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/BlockRegistrar.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;
 
 /**
  * Registers Gutenberg dynamic-block wrappers for the front-end shortcodes so
@@ -28,6 +29,7 @@ class BlockRegistrar {
 		private LoginPage $loginPage,
 		private RegistrationPage $registrationPage,
 		private GroupClassPage $groupClassPage,
+		private FamilyPage $familyPage,
 	) {}
 
 	public function register(): void {
@@ -137,6 +139,15 @@ class BlockRegistrar {
 					],
 				],
 			],
+			'us-scheduler/family'           => [
+				'render'     => [ $this, 'renderFamily' ],
+				'attributes' => [
+					'loginPageId' => [
+						'type'    => 'number',
+						'default' => 0,
+					],
+				],
+			],
 		];
 	}
 
@@ -184,6 +195,15 @@ class BlockRegistrar {
 		return BlockPreview::groupClasses( Val::int( $attributes['offeringId'] ?? 0 ) > 0 );
 	}
 
+	/**
+	 * Renders the family (manage-children) block.
+	 *
+	 * @param array<string, mixed> $attributes Block attributes.
+	 */
+	public function renderFamily( array $attributes = [] ): string {
+		return $this->isEditorPreview() ? BlockPreview::family() : $this->familyPage->render( $attributes );
+	}
+
 	/**
 	 * Server-side auto-redirect for blocks that opt in via their autoRedirect
 	 * attribute: logged-out visitors on a page containing the booking block