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/Payment.php

@@ -0,0 +1,92 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Payment;
+
+class Payment {
+
+	public const METHOD_CARD      = 'card';
+	public const METHOD_ETRANSFER = 'etransfer';
+	public const METHOD_COMP      = 'comp';
+
+	/**
+	 * All valid payment methods.
+	 *
+	 * @var list<string>
+	 */
+	public const VALID_METHODS = [ self::METHOD_CARD, self::METHOD_ETRANSFER, self::METHOD_COMP ];
+
+	public const STATUS_PENDING  = 'pending';
+	public const STATUS_PAID     = 'paid';
+	public const STATUS_FAILED   = 'failed';
+	public const STATUS_REFUNDED = 'refunded';
+
+	/**
+	 * All valid payment statuses.
+	 *
+	 * @var list<string>
+	 */
+	public const VALID_STATUSES = [ self::STATUS_PENDING, self::STATUS_PAID, self::STATUS_FAILED, self::STATUS_REFUNDED ];
+
+	public const REG_LESSON     = 'lesson';
+	public const REG_ENROLLMENT = 'enrollment';
+
+	public function __construct(
+		public readonly int $studentId,
+		public readonly int $instructorId,
+		public readonly string $registrationType,
+		public readonly int $registrationId,
+		public readonly float $amount,
+		public readonly string $currency = 'CAD',
+		public readonly string $method = self::METHOD_ETRANSFER,
+		public readonly string $status = self::STATUS_PENDING,
+		public readonly ?string $stripePaymentIntentId = null,
+		public readonly ?string $receiptNumber = null,
+		public readonly ?string $receiptSentAt = null,
+		public readonly ?string $paidAt = null,
+		public readonly ?int $id = null,
+	) {}
+
+	public static function fromRow( object $row ): self {
+		return new self(
+			studentId:             (int) $row->student_id,
+			instructorId:          (int) $row->instructor_id,
+			registrationType:      $row->registration_type,
+			registrationId:        (int) $row->registration_id,
+			amount:                (float) $row->amount,
+			currency:              $row->currency,
+			method:                $row->method,
+			status:                $row->status,
+			stripePaymentIntentId: $row->stripe_payment_intent_id,
+			receiptNumber:         $row->receipt_number,
+			receiptSentAt:         $row->receipt_sent_at,
+			paidAt:                $row->paid_at,
+			id:                    (int) $row->id,
+		);
+	}
+
+	public function isPaid(): bool {
+		return self::STATUS_PAID === $this->status;
+	}
+
+	/**
+	 * Returns a plain array representation of the payment.
+	 *
+	 * @return array<string, mixed>
+	 */
+	public function toArray(): array {
+		return [
+			'id'                => $this->id,
+			'student_id'        => $this->studentId,
+			'instructor_id'     => $this->instructorId,
+			'registration_type' => $this->registrationType,
+			'registration_id'   => $this->registrationId,
+			'amount'            => $this->amount,
+			'currency'          => $this->currency,
+			'method'            => $this->method,
+			'status'            => $this->status,
+			'receipt_number'    => $this->receiptNumber,
+			'paid_at'           => $this->paidAt,
+		];
+	}
+}