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]>

tests/Unit/GroupClass/GroupClassPageTest.php

@@ -4,20 +4,29 @@ declare(strict_types=1);
 namespace Unsupervised\Schedular\Tests\Unit\GroupClass;
 
 use Brain\Monkey\Functions;
+use Mockery;
 use Unsupervised\Schedular\GroupClass\GroupClassPage;
+use Unsupervised\Schedular\Guardian\GuardianService;
 use Unsupervised\Schedular\Tests\Unit\TestCase;
 
 class GroupClassPageTest extends TestCase
 {
     private GroupClassPage $page;
+    private GuardianService&Mockery\MockInterface $guardians;
 
     protected function setUp(): void
     {
         parent::setUp();
 
-        $this->page = new GroupClassPage();
+        $this->guardians = Mockery::mock(GuardianService::class);
+        $this->guardians->shouldReceive('bookableStudents')->andReturn(
+            [['id' => 3, 'name' => 'Ada', 'is_self' => true]]
+        )->byDefault();
+
+        $this->page = new GroupClassPage($this->guardians);
 
         Functions\when('is_user_logged_in')->justReturn(true);
+        Functions\when('get_current_user_id')->justReturn(3);
         Functions\when('current_user_can')->justReturn(true);
         Functions\when('wp_enqueue_style')->justReturn(null);
         Functions\when('wp_enqueue_script')->justReturn(null);