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 <[email protected]>

src/Offering/Offering.php

@@ -39,6 +39,7 @@ class Offering {
 		public readonly ?string $termStart = null,
 		public readonly ?string $termEnd = null,
 		public readonly ?string $scheduleNote = null,
+		public readonly ?string $etransferEmail = null,
 		public readonly bool $isActive = true,
 		public readonly ?int $id = null,
 	) {}
@@ -58,6 +59,7 @@ class Offering {
 			termStart:       $row->term_start,
 			termEnd:         $row->term_end,
 			scheduleNote:    $row->schedule_note,
+			etransferEmail:  $row->etransfer_email,
 			isActive:        (bool) $row->is_active,
 			id:              (int) $row->id,
 		);
@@ -84,6 +86,7 @@ class Offering {
 			'term_start'       => $this->termStart,
 			'term_end'         => $this->termEnd,
 			'schedule_note'    => $this->scheduleNote,
+			'etransfer_email'  => $this->etransferEmail,
 			'is_active'        => $this->isActive,
 		];
 	}