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/offerings.php

@@ -59,6 +59,10 @@ if (! defined('ABSPATH')) {
                 <th><label for="schedule_note"><?php esc_html_e('Schedule note', 'unsupervised-schedular'); ?></label></th>
                 <td><input type="text" name="schedule_note" id="schedule_note" class="regular-text" placeholder="<?php esc_attr_e('e.g. Tuesdays 4:00pm', 'unsupervised-schedular'); ?>"></td>
             </tr>
+            <tr>
+                <th><label for="etransfer_email"><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="email" name="etransfer_email" id="etransfer_email" class="regular-text" placeholder="<?php esc_attr_e('Overrides the studio default', 'unsupervised-schedular'); ?>"></td>
+            </tr>
         </table>
         <?php submit_button(esc_html__('Add Offering', 'unsupervised-schedular')); ?>
     </form>