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/lessons.php

@@ -5,12 +5,12 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-/** @var list<\Unsupervised\Schedular\Model\Lesson> $lessons */
+/** @var list<array{student: string, instructor: string, slot_id: int, status: string, notes: string, payment_id: int, etransfer_email: string, etransfer_editable: bool}> $rows */
 ?>
 <div class="wrap">
     <h1><?php esc_html_e('Lessons', 'unsupervised-schedular'); ?></h1>
 
-    <?php if (empty($lessons)) : ?>
+    <?php if (empty($rows)) : ?>
         <p><?php esc_html_e('No upcoming lessons.', 'unsupervised-schedular'); ?></p>
     <?php else : ?>
         <table class="wp-list-table widefat fixed striped">
@@ -20,21 +20,31 @@ if (! defined('ABSPATH')) {
                     <th><?php esc_html_e('Instructor', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Slot ID', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Status', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Notes', 'unsupervised-schedular'); ?></th>
                 </tr>
             </thead>
             <tbody>
-                <?php foreach ($lessons as $lesson) : ?>
-                    <?php
-                    $student    = get_userdata($lesson->studentId);
-                    $instructor = get_userdata($lesson->instructorId);
-                    ?>
+                <?php foreach ($rows as $row) : ?>
                     <tr>
-                        <td><?php echo esc_html($student ? $student->display_name : (string) $lesson->studentId); ?></td>
-                        <td><?php echo esc_html($instructor ? $instructor->display_name : (string) $lesson->instructorId); ?></td>
-                        <td><?php echo esc_html((string) $lesson->slotId); ?></td>
-                        <td><?php echo esc_html($lesson->status); ?></td>
-                        <td><?php echo esc_html($lesson->notes ?? ''); ?></td>
+                        <td><?php echo esc_html($row['student']); ?></td>
+                        <td><?php echo esc_html($row['instructor']); ?></td>
+                        <td><?php echo esc_html((string) $row['slot_id']); ?></td>
+                        <td><?php echo esc_html($row['status']); ?></td>
+                        <td>
+                            <?php if ($row['etransfer_editable']) : ?>
+                                <form method="post" style="display:flex; gap:4px;">
+                                    <?php wp_nonce_field('usc_lesson_action'); ?>
+                                    <input type="hidden" name="usc_action" value="set_etransfer">
+                                    <input type="hidden" name="payment_id" value="<?php echo esc_attr((string) $row['payment_id']); ?>">
+                                    <input type="email" name="etransfer_email" value="<?php echo esc_attr($row['etransfer_email']); ?>">
+                                    <button type="submit" class="button button-small"><?php esc_html_e('Save', 'unsupervised-schedular'); ?></button>
+                                </form>
+                            <?php else : ?>
+                                <?php echo esc_html('' !== $row['etransfer_email'] ? $row['etransfer_email'] : '—'); ?>
+                            <?php endif; ?>
+                        </td>
+                        <td><?php echo esc_html($row['notes']); ?></td>
                     </tr>
                 <?php endforeach; ?>
             </tbody>