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/BillingMethodResolver.php

@@ -0,0 +1,34 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Payment;
+
+/**
+ * Resolves the billing method for a student: a per-student override if set,
+ * otherwise the studio default — card when Stripe is configured, e-transfer when
+ * it is not.
+ */
+class BillingMethodResolver {
+
+	public const META_METHOD = 'us_payment_method';
+
+	public function __construct( private StudioSettings $settings ) {}
+
+	public function resolve( int $studentId ): string {
+		$override = (string) get_user_meta( $studentId, self::META_METHOD, true );
+		if ( in_array( $override, Payment::VALID_METHODS, true ) ) {
+			return $override;
+		}
+
+		return $this->defaultMethod();
+	}
+
+	/**
+	 * The studio default when a student has no explicit override.
+	 */
+	public function defaultMethod(): string {
+		return $this->settings->isStripeConfigured()
+			? Payment::METHOD_CARD
+			: Payment::METHOD_ETRANSFER;
+	}
+}