Add e-transfer destination email (studio default + offering/booking overrides)

The e-transfer destination is resolved at booking time (offering override ->
studio default) and frozen onto the payment, so each record keeps where the
student was directed. It can then be corrected per booking.

- StudioSettings: us_etransfer_email option + a Default e-transfer email field
  on the Studio Settings page.
- Offering: etransfer_email column/field (instructor override) across VO, repo,
  REST endpoint, admin controller, and form.
- Payment: etransfer_email column on the payment (frozen record) +
  PaymentRepository::updateEtransferEmail; PaymentService freezes it from the
  offering override or studio default at creation; booking/enrolment pass the
  offering override.
- My Lessons: instructors edit the e-transfer email per pending lesson payment
  (ownership-checked).
- Payments queue: studio admin can correct the email at confirmation (for when
  a student sends it to the wrong place).
- Docs updated.

Tests: Payment/Offering rows + PaymentService freezing. composer test (148),
cs, and PHPStan level 6 all pass.

Refs #7

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/admin/payments.php

@@ -5,11 +5,11 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-/** @var list<array{id: int, student: string, amount: string, method: string, for: string}> $rows */
+/** @var list<array{id: int, student: string, amount: string, method: string, for: string, etransfer_email: string}> $rows */
 ?>
 <div class="wrap">
     <h1><?php esc_html_e('Payments', 'unsupervised-schedular'); ?></h1>
-    <p class="description"><?php esc_html_e('Pending payments awaiting confirmation. Marking one received confirms the booking and emails a receipt.', 'unsupervised-schedular'); ?></p>
+    <p class="description"><?php esc_html_e('Pending payments awaiting confirmation. Marking one received confirms the booking and emails a receipt. You can correct the e-transfer email here if the student sent it elsewhere.', 'unsupervised-schedular'); ?></p>
 
     <?php if (empty($rows)) : ?>
         <p><?php esc_html_e('No pending payments.', 'unsupervised-schedular'); ?></p>
@@ -21,26 +21,28 @@ if (! defined('ABSPATH')) {
                     <th><?php esc_html_e('For', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Amount', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Method', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Actions', 'unsupervised-schedular'); ?></th>
                 </tr>
             </thead>
             <tbody>
                 <?php foreach ($rows as $row) : ?>
                     <tr>
-                        <td><?php echo esc_html($row['student']); ?></td>
-                        <td><?php echo esc_html($row['for']); ?></td>
-                        <td><?php echo esc_html($row['amount']); ?></td>
-                        <td><?php echo esc_html($row['method']); ?></td>
-                        <td>
-                            <form method="post" style="display:inline;">
-                                <?php wp_nonce_field('usc_payment_action'); ?>
-                                <input type="hidden" name="usc_action" value="mark_paid">
-                                <input type="hidden" name="payment_id" value="<?php echo esc_attr((string) $row['id']); ?>">
+                        <form method="post">
+                            <?php wp_nonce_field('usc_payment_action'); ?>
+                            <input type="hidden" name="usc_action" value="mark_paid">
+                            <input type="hidden" name="payment_id" value="<?php echo esc_attr((string) $row['id']); ?>">
+                            <td><?php echo esc_html($row['student']); ?></td>
+                            <td><?php echo esc_html($row['for']); ?></td>
+                            <td><?php echo esc_html($row['amount']); ?></td>
+                            <td><?php echo esc_html($row['method']); ?></td>
+                            <td><input type="email" name="etransfer_email" class="regular-text" value="<?php echo esc_attr($row['etransfer_email']); ?>"></td>
+                            <td>
                                 <button type="submit" class="button button-small button-primary">
                                     <?php esc_html_e('Mark received', 'unsupervised-schedular'); ?>
                                 </button>
-                            </form>
-                        </td>
+                            </td>
+                        </form>
                     </tr>
                 <?php endforeach; ?>
             </tbody>