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/Payment/PaymentRepository.php

@@ -23,18 +23,29 @@ class PaymentRepository {
 				'currency'                 => $payment->currency,
 				'method'                   => $payment->method,
 				'status'                   => $payment->status,
+				'etransfer_email'          => $payment->etransferEmail,
 				'stripe_payment_intent_id' => $payment->stripePaymentIntentId,
 				'receipt_number'           => $payment->receiptNumber,
 				'receipt_sent_at'          => $payment->receiptSentAt,
 				'paid_at'                  => $payment->paidAt,
 				'created_at'               => current_time( 'mysql' ),
 			],
-			[ '%d', '%d', '%s', '%d', '%f', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' ]
+			[ '%d', '%d', '%s', '%d', '%f', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' ]
 		);
 
 		return $this->db->insert_id;
 	}
 
+	public function updateEtransferEmail( int $id, ?string $email ): bool {
+		return false !== $this->db->update(
+			$this->table,
+			[ 'etransfer_email' => $email ],
+			[ 'id' => $id ],
+			[ '%s' ],
+			[ '%d' ]
+		);
+	}
+
 	public function findById( int $id ): ?Payment {
 		$row = $this->db->get_row(
 			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )