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/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,
 		];
 	}

src/Offering/OfferingController.php

@@ -77,6 +77,7 @@ class OfferingController {
 				allowWeekly:     isset( $_POST['allow_weekly'] ),
 				capacity:        $capacity > 0 ? $capacity : null,
 				scheduleNote:    $this->nullableText( sanitize_text_field( wp_unslash( $_POST['schedule_note'] ?? '' ) ) ),
+				etransferEmail:  $this->nullableText( sanitize_email( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) ),
 			)
 		);
 		// phpcs:enable WordPress.Security.NonceVerification.Missing

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;
 	}

src/Offering/OfferingRepository.php

@@ -14,11 +14,11 @@ class OfferingRepository {
 	/**
 	 * Column formats aligned to {@see columns()} (instructor_id, kind, title,
 	 * description, duration_minutes, price, currency, billing_mode, allow_weekly,
-	 * capacity, term_start, term_end, schedule_note, is_active).
+	 * capacity, term_start, term_end, schedule_note, etransfer_email, is_active).
 	 *
 	 * @var list<string>
 	 */
-	private const COLUMN_FORMATS = [ '%d', '%s', '%s', '%s', '%d', '%f', '%s', '%s', '%d', '%d', '%s', '%s', '%s', '%d' ];
+	private const COLUMN_FORMATS = [ '%d', '%s', '%s', '%s', '%d', '%f', '%s', '%s', '%d', '%d', '%s', '%s', '%s', '%s', '%d' ];
 
 	public function insert( Offering $offering ): int {
 		$this->db->insert(
@@ -60,6 +60,7 @@ class OfferingRepository {
 			'term_start'       => $offering->termStart,
 			'term_end'         => $offering->termEnd,
 			'schedule_note'    => $offering->scheduleNote,
+			'etransfer_email'  => $offering->etransferEmail,
 			'is_active'        => $offering->isActive ? 1 : 0,
 		];
 	}