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>

src/Payment/Payment.php

@@ -40,6 +40,7 @@ class Payment {
 		public readonly string $currency = 'CAD',
 		public readonly string $method = self::METHOD_ETRANSFER,
 		public readonly string $status = self::STATUS_PENDING,
+		public readonly ?string $etransferEmail = null,
 		public readonly ?string $stripePaymentIntentId = null,
 		public readonly ?string $receiptNumber = null,
 		public readonly ?string $receiptSentAt = null,
@@ -57,6 +58,7 @@ class Payment {
 			currency:              $row->currency,
 			method:                $row->method,
 			status:                $row->status,
+			etransferEmail:        $row->etransfer_email,
 			stripePaymentIntentId: $row->stripe_payment_intent_id,
 			receiptNumber:         $row->receipt_number,
 			receiptSentAt:         $row->receipt_sent_at,
@@ -80,6 +82,7 @@ class Payment {
 			'student_id'        => $this->studentId,
 			'instructor_id'     => $this->instructorId,
 			'registration_type' => $this->registrationType,
+			'etransfer_email'   => $this->etransferEmail,
 			'registration_id'   => $this->registrationId,
 			'amount'            => $this->amount,
 			'currency'          => $this->currency,