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>

tests/Unit/Offering/OfferingRepositoryTest.php

@@ -176,6 +176,7 @@ class OfferingRepositoryTest extends TestCase
             'term_start'       => null,
             'term_end'         => null,
             'schedule_note'    => null,
+            'etransfer_email'  => null,
             'is_active'        => '1',
         ];
     }

tests/Unit/Offering/OfferingTest.php

@@ -59,6 +59,7 @@ class OfferingTest extends TestCase
             'term_start'       => '2026-09-01',
             'term_end'         => '2027-06-30',
             'schedule_note'    => 'Tuesdays 4:00pm',
+            'etransfer_email'  => null,
             'is_active'        => '1',
         ];