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 <[email protected]>

src/Payment/PaymentRepository.php

@@ -0,0 +1,114 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Payment;
+
+class PaymentRepository {
+
+	private string $table;
+
+	public function __construct( private \wpdb $db ) {
+		$this->table = $db->prefix . 'us_payments';
+	}
+
+	public function insert( Payment $payment ): int {
+		$this->db->insert(
+			$this->table,
+			[
+				'student_id'               => $payment->studentId,
+				'instructor_id'            => $payment->instructorId,
+				'registration_type'        => $payment->registrationType,
+				'registration_id'          => $payment->registrationId,
+				'amount'                   => $payment->amount,
+				'currency'                 => $payment->currency,
+				'method'                   => $payment->method,
+				'status'                   => $payment->status,
+				'stripe_payment_intent_id' => $payment->stripePaymentIntentId,
+				'receipt_number'           => $payment->receiptNumber,
+				'receipt_sent_at'          => $payment->receiptSentAt,
+				'paid_at'                  => $payment->paidAt,
+				'created_at'               => current_time( 'mysql' ),
+			],
+			[ '%d', '%d', '%s', '%d', '%f', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' ]
+		);
+
+		return $this->db->insert_id;
+	}
+
+	public function findById( int $id ): ?Payment {
+		$row = $this->db->get_row(
+			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )
+		);
+
+		return $row ? Payment::fromRow( $row ) : null;
+	}
+
+	public function findByRegistration( string $registrationType, int $registrationId ): ?Payment {
+		$row = $this->db->get_row(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE registration_type = %s AND registration_id = %d ORDER BY id DESC LIMIT 1",
+				$registrationType,
+				$registrationId
+			)
+		);
+
+		return $row ? Payment::fromRow( $row ) : null;
+	}
+
+	/**
+	 * Pending payments, newest first (studio-admin confirmation queue).
+	 *
+	 * @return list<Payment>
+	 */
+	public function findPending(): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE status = %s ORDER BY created_at DESC",
+				Payment::STATUS_PENDING
+			)
+		);
+
+		return array_map( Payment::fromRow( ... ), $rows ?? [] );
+	}
+
+	/**
+	 * Mark a payment paid, stamping the paid time and receipt number.
+	 */
+	public function markPaid( int $id, string $receiptNumber ): bool {
+		return false !== $this->db->update(
+			$this->table,
+			[
+				'status'         => Payment::STATUS_PAID,
+				'paid_at'        => current_time( 'mysql' ),
+				'receipt_number' => $receiptNumber,
+			],
+			[ 'id' => $id ],
+			[ '%s', '%s', '%s' ],
+			[ '%d' ]
+		);
+	}
+
+	public function markReceiptSent( int $id ): bool {
+		return false !== $this->db->update(
+			$this->table,
+			[ 'receipt_sent_at' => current_time( 'mysql' ) ],
+			[ 'id' => $id ],
+			[ '%s' ],
+			[ '%d' ]
+		);
+	}
+
+	public function updateStatus( int $id, string $status ): bool {
+		if ( ! in_array( $status, Payment::VALID_STATUSES, true ) ) {
+			return false;
+		}
+
+		return (bool) $this->db->update(
+			$this->table,
+			[ 'status' => $status ],
+			[ 'id' => $id ],
+			[ '%s' ],
+			[ '%d' ]
+		);
+	}
+}