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/Auth/StudentController.php

@@ -9,6 +9,8 @@ use Unsupervised\Schedular\Booking\Lesson;
 use Unsupervised\Schedular\GroupClass\Enrollment;
 use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Payment\BillingMethodResolver;
+use Unsupervised\Schedular\Payment\Payment;
 
 class StudentController {
 
@@ -17,6 +19,7 @@ class StudentController {
 		private AvailabilityRepository $availability,
 		private OfferingRepository $offerings,
 		private EnrollmentRepository $enrollments,
+		private BillingMethodResolver $resolver,
 	) {}
 
 	public function renderPage(): void {
@@ -56,6 +59,21 @@ class StudentController {
 	}
 
 	private function renderDetail( \WP_User $student ): void {
+		$canBilling = current_user_can( RoleManager::CAP_MANAGE_BILLING );
+
+		if ( $canBilling && isset( $_POST['usc_action'] ) && check_admin_referer( 'usc_student_billing' ) ) {
+			// phpcs:ignore WordPress.Security.NonceVerification.Missing -- nonce checked above.
+			$method = sanitize_key( wp_unslash( $_POST['payment_method'] ?? '' ) );
+			if ( in_array( $method, Payment::VALID_METHODS, true ) ) {
+				update_user_meta( (int) $student->ID, BillingMethodResolver::META_METHOD, $method );
+			} else {
+				delete_user_meta( (int) $student->ID, BillingMethodResolver::META_METHOD );
+			}
+		}
+
+		$billingOverride = (string) get_user_meta( (int) $student->ID, BillingMethodResolver::META_METHOD, true );
+		$billingDefault  = $this->resolver->defaultMethod();
+
 		$now  = current_time( 'mysql' );
 		$rows = array_map(
 			fn( Lesson $lesson ): array => $this->lessonRow( $lesson ),