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/StudioSettings.php

@@ -7,10 +7,11 @@ use Unsupervised\Schedular\Auth\RoleManager;
 
 class StudioSettings {
 
-	public const OPT_PUBLISHABLE = 'us_stripe_publishable_key';
-	public const OPT_SECRET      = 'us_stripe_secret_key';
-	public const OPT_MODE        = 'us_stripe_mode';
-	public const OPT_CURRENCY    = 'us_currency';
+	public const OPT_PUBLISHABLE     = 'us_stripe_publishable_key';
+	public const OPT_SECRET          = 'us_stripe_secret_key';
+	public const OPT_MODE            = 'us_stripe_mode';
+	public const OPT_CURRENCY        = 'us_currency';
+	public const OPT_ETRANSFER_EMAIL = 'us_etransfer_email';
 
 	public function publishableKey(): string {
 		return (string) get_option( self::OPT_PUBLISHABLE, '' );
@@ -30,6 +31,14 @@ class StudioSettings {
 		return '' !== $currency ? strtoupper( $currency ) : 'CAD';
 	}
 
+	/**
+	 * The studio-default e-transfer destination email (used when an offering has
+	 * no override).
+	 */
+	public function etransferEmail(): string {
+		return (string) get_option( self::OPT_ETRANSFER_EMAIL, '' );
+	}
+
 	/**
 	 * Whether Stripe is configured. When false the platform falls back to
 	 * e-transfer billing and card processing is unavailable.
@@ -51,6 +60,7 @@ class StudioSettings {
 		$secretKey        = $this->secretKey();
 		$mode             = $this->mode();
 		$currency         = $this->currency();
+		$etransferEmail   = $this->etransferEmail();
 		$stripeConfigured = $this->isStripeConfigured();
 
 		include USC_PLUGIN_DIR . 'templates/admin/settings.php';
@@ -64,6 +74,7 @@ class StudioSettings {
 		update_option( self::OPT_SECRET, sanitize_text_field( wp_unslash( $_POST['secret_key'] ?? '' ) ) );
 		update_option( self::OPT_MODE, 'live' === $mode ? 'live' : 'test' );
 		update_option( self::OPT_CURRENCY, strtoupper( sanitize_text_field( wp_unslash( $_POST['currency'] ?? 'CAD' ) ) ) );
+		update_option( self::OPT_ETRANSFER_EMAIL, sanitize_email( wp_unslash( $_POST['etransfer_email'] ?? '' ) ) );
 		// phpcs:enable WordPress.Security.NonceVerification.Missing
 	}
 }