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/Booking/BookingPageTest.php

@@ -4,17 +4,26 @@ declare(strict_types=1);
 namespace Unsupervised\Schedular\Tests\Unit\Booking;
 
 use Brain\Monkey\Functions;
+use Mockery;
 use Unsupervised\Schedular\Booking\BookingPage;
+use Unsupervised\Schedular\Guardian\GuardianService;
 use Unsupervised\Schedular\Tests\Unit\TestCase;
 
 class BookingPageTest extends TestCase
 {
     private BookingPage $page;
+    private GuardianService&Mockery\MockInterface $guardians;
 
     protected function setUp(): void
     {
         parent::setUp();
-        $this->page = new BookingPage();
+        $this->guardians = Mockery::mock(GuardianService::class);
+        // Most cases are a single-student account: one bookable person, no picker.
+        $this->guardians->shouldReceive('bookableStudents')->andReturn(
+            [['id' => 3, 'name' => 'Ada', 'is_self' => true]]
+        )->byDefault();
+
+        $this->page = new BookingPage($this->guardians);
     }
 
     /**
@@ -171,4 +180,35 @@ class BookingPageTest extends TestCase
 
         self::assertSame('https://example.com/wp-login.php', $this->page->loginUrl(5));
     }
+
+    /**
+     * A single-student account gets a one-entry list, which the script renders
+     * as no picker at all.
+     */
+    public function testStudentListIsEmbeddedForTheScript(): void
+    {
+        $html = $this->renderForStudent([]);
+
+        self::assertStringContainsString('data-students=', $html);
+        self::assertStringContainsString('"is_self":true', $html);
+    }
+
+    /**
+     * Children lead the embedded list, so the picker's default selection is a
+     * child rather than the parent.
+     */
+    public function testGuardianListLeadsWithChildren(): void
+    {
+        $this->guardians->shouldReceive('bookableStudents')->with(3)->andReturn([
+            ['id' => 42, 'name' => 'Ada', 'is_self' => false],
+            ['id' => 3, 'name' => 'Grace', 'is_self' => true],
+        ]);
+
+        $html = $this->renderForStudent([]);
+
+        $students = json_decode(html_entity_decode((string) preg_replace('/.*data-students="([^"]*)".*/s', '$1', $html)), true);
+
+        self::assertSame([42, 3], array_column((array) $students, 'id'));
+        self::assertFalse($students[0]['is_self']);
+    }
 }