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]>

templates/admin/student-detail.php

@@ -15,8 +15,12 @@ if (! defined('ABSPATH')) {
  * @var list<array{question: string, answer: string, context: string}> $intake
  * @var list<array{created_at: string, context: string, method: string, status: string, amount: float, tax_amount: float, total: float, currency: string, receipt: string}> $payments
  * @var list<array{created_at: string, amount: float, remaining: float, currency: string, reason: string, status: string}> $credits
- * @var float  $creditBalance
+ * @var float  $creditBalance Balance of the account that settles this student's charges — the guardian's for a child.
  * @var string $creditCurrency
+ * @var array{id: int, name: string, email: string}|null $guardian The parent/guardian who books for this student, or null when they book for themselves.
+ * @var list<array{id: int, name: string, date_of_birth: string, relationship: string}> $children Children this student books for.
+ * @var array{id: int, name: string, email: string} $payer Who is billed for this student — themselves, or their guardian.
+ * @var string $pageSlug
  * @var string $backUrl
  * @var bool   $canBilling
  * @var string $billingOverride
@@ -105,6 +109,37 @@ $renderLessons = static function (array $rows, bool $withActions = false): void
         <?php submit_button(esc_html__('Save account details', 'unsupervised-schedular'), 'secondary', 'submit', false); ?>
     </form>
 
+    <?php if ($guardian !== null || ! empty($children)) : ?>
+        <h2><?php esc_html_e('Family', 'unsupervised-schedular'); ?></h2>
+        <?php $detailUrl = static fn(int $id): string => add_query_arg(['page' => $pageSlug, 'student_id' => $id], admin_url('admin.php')); ?>
+        <?php if ($guardian !== null) : ?>
+            <p>
+                <?php
+                printf(
+                    /* translators: 1: linked name of the parent/guardian, 2: their email address. */
+                    esc_html__('Books and pays through %1$s (%2$s).', 'unsupervised-schedular'),
+                    '<a href="' . esc_url($detailUrl($guardian['id'])) . '">' . esc_html($guardian['name']) . '</a>',
+                    esc_html($guardian['email'])
+                );
+                ?>
+            </p>
+            <p class="description"><?php esc_html_e('This is a child account: it has no login of its own, and its email address is a placeholder that cannot receive mail.', 'unsupervised-schedular'); ?></p>
+        <?php endif; ?>
+        <?php if (! empty($children)) : ?>
+            <p><?php esc_html_e('Books and pays for:', 'unsupervised-schedular'); ?></p>
+            <ul class="ul-disc">
+                <?php foreach ($children as $child) : ?>
+                    <li>
+                        <a href="<?php echo esc_url($detailUrl($child['id'])); ?>"><?php echo esc_html($child['name']); ?></a>
+                        <?php if ($child['date_of_birth'] !== '') : ?>
+                            <span class="description"><?php echo esc_html($child['date_of_birth']); ?></span>
+                        <?php endif; ?>
+                    </li>
+                <?php endforeach; ?>
+            </ul>
+        <?php endif; ?>
+    <?php endif; ?>
+
     <h2><?php esc_html_e('Registration Information', 'unsupervised-schedular'); ?></h2>
     <?php if (empty($registrationInfo)) : ?>
         <p><?php esc_html_e('No registration questions are configured.', 'unsupervised-schedular'); ?></p>
@@ -253,6 +288,17 @@ $renderLessons = static function (array $rows, bool $withActions = false): void
                 '<strong>' . esc_html(number_format_i18n($creditBalance, 2) . ' ' . $creditCurrency) . '</strong>'
             );
             ?>
+            <?php if ($payer['id'] !== (int) $student->ID) : ?>
+                <span class="description">
+                    <?php
+                    printf(
+                        /* translators: %s: name of the parent/guardian whose account holds the balance. */
+                        esc_html__('Held on %s’s account — the family shares one balance.', 'unsupervised-schedular'),
+                        esc_html($payer['name'])
+                    );
+                    ?>
+                </span>
+            <?php endif; ?>
             <span class="description"><?php esc_html_e('Credit from cancelled paid lessons is applied automatically to upcoming scheduled billing.', 'unsupervised-schedular'); ?></span>
         </p>
         <?php if (! empty($credits)) : ?>