Add payments foundation (e-transfer/comp, Stripe config, receipts)

Implements the payments foundation for #7. Without Stripe credentials
everything works on e-transfer (pending payment confirmed by a studio
admin); when Stripe keys are configured the default flips to credit card.
Per-student override (card/etransfer/comp) is set on the student detail.

- Schema: us_payments (amount DECIMAL dollars, method, status, receipt,
  stripe intent id).
- src/Payment/: Payment VO, PaymentRepository, StudioSettings (Stripe
  options + isStripeConfigured + settings page), BillingMethodResolver
  (per-student override; default card if configured else etransfer),
  ReceiptMailer, PaymentService (create at registration, link payment_id,
  comp->paid+confirm, markPaid->confirm+receipt), PaymentController
  (e-transfer confirmation queue), PaymentEndpoint (PATCH /payments/{id}).
- Booking + enrolment create the payment from the offering price; comp
  auto-confirms the lesson; setPaymentId on both repositories.
- Admin: Studio Settings + Payments menus (manage_billing); per-student
  billing method on the student detail page.
- Docs: payments.md + README updated.

Deferred to a follow-up: the live Stripe card charge (PaymentIntent +
Stripe.js Elements + webhook + stripe/stripe-php). Until then a card
payment is created pending and confirmed like an e-transfer.

Tests: tests/Unit/Payment/ (VO, repository, resolver, service, mailer).
composer test (147), cs, and PHPStan level 6 all pass.

Refs #7

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Payment/ReceiptMailer.php

@@ -0,0 +1,33 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Payment;
+
+class ReceiptMailer {
+
+	/**
+	 * Email a numbered receipt to the student. Returns false when there is no
+	 * recipient.
+	 */
+	public function send( Payment $payment, ?\WP_User $student ): bool {
+		if ( null === $student || '' === (string) $student->user_email ) {
+			return false;
+		}
+
+		$subject = sprintf(
+			/* translators: %s: receipt number */
+			__( 'Payment receipt %s', 'unsupervised-schedular' ),
+			(string) $payment->receiptNumber
+		);
+
+		$body = sprintf(
+			/* translators: 1: amount, 2: currency, 3: receipt number */
+			__( "Thank you. We have recorded your payment of %1\$s %2\$s.\n\nReceipt: %3\$s", 'unsupervised-schedular' ),
+			number_format( $payment->amount, 2 ),
+			$payment->currency,
+			(string) $payment->receiptNumber
+		);
+
+		return (bool) wp_mail( $student->user_email, $subject, $body );
+	}
+}