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/Offering/OfferingEndpoint.php

@@ -95,6 +95,7 @@ class OfferingEndpoint {
 			termStart:       $this->nullableText( $request->get_param( 'term_start' ) ),
 			termEnd:         $this->nullableText( $request->get_param( 'term_end' ) ),
 			scheduleNote:    $this->nullableText( $request->get_param( 'schedule_note' ) ),
+			etransferEmail:  $this->nullableEmail( $request->get_param( 'etransfer_email' ) ),
 			isActive:        null === $request->get_param( 'is_active' ) ? true : (bool) $request->get_param( 'is_active' ),
 		);
 
@@ -139,6 +140,7 @@ class OfferingEndpoint {
 			termStart:       $request->has_param( 'term_start' ) ? $this->nullableText( $request->get_param( 'term_start' ) ) : $existing->termStart,
 			termEnd:         $request->has_param( 'term_end' ) ? $this->nullableText( $request->get_param( 'term_end' ) ) : $existing->termEnd,
 			scheduleNote:    $request->has_param( 'schedule_note' ) ? $this->nullableText( $request->get_param( 'schedule_note' ) ) : $existing->scheduleNote,
+			etransferEmail:  $request->has_param( 'etransfer_email' ) ? $this->nullableEmail( $request->get_param( 'etransfer_email' ) ) : $existing->etransferEmail,
 			isActive:        $request->has_param( 'is_active' ) ? (bool) $request->get_param( 'is_active' ) : $existing->isActive,
 			id:              $id,
 		);
@@ -186,6 +188,12 @@ class OfferingEndpoint {
 		return max( 0.0, (float) $value );
 	}
 
+	private function nullableEmail( mixed $value ): ?string {
+		$email = sanitize_email( (string) $value );
+
+		return '' !== $email ? $email : null;
+	}
+
 	private function nullableInt( mixed $value ): ?int {
 		return ( null === $value || '' === $value ) ? null : (int) $value;
 	}