Add live Stripe card charges (PaymentIntent + Elements + webhook)

Completes the deferred half of payments: real credit-card processing on
top of the existing ledger/e-transfer/comp foundation.

- StripeGateway wraps stripe/stripe-php: creates idempotent PaymentIntents
  (amount in cents, registration ids in metadata) and verifies webhook
  signatures. Stripe calls sit behind protected seams for unit testing.
- PaymentService::createIntent resolves the client-side step for a new
  registration (card → client secret; e-transfer → display data; comp →
  none) with caller-ownership enforcement.
- PaymentService::handleWebhook finalises a payment exactly once on
  payment_intent.succeeded (mark paid → confirm → receipt) and marks it
  failed on payment_intent.payment_failed.
- PaymentEndpoint: POST /payments/intent (book_lesson) and public,
  signature-verified POST /payments/webhook.
- PaymentRepository: setStripeIntentId / findByStripeIntentId.
- StudioSettings: us_stripe_webhook_secret option, with the webhook URL
  and required events surfaced on the settings page.
- Front end: shared payment.js mounts Stripe Payment Elements and confirms
  the card (or shows e-transfer instructions); Stripe.js enqueued only when
  configured. Wired into booking and group-class flows.

Tests: new StripeGatewayTest; PaymentService card-intent + webhook cases;
repository coverage. composer test/lint/cs all green.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Payment/PaymentRepository.php

@@ -38,6 +38,31 @@ class PaymentRepository {
 		return $this->db->insert_id;
 	}
 
+	/**
+	 * Attach the Stripe PaymentIntent id created for a card payment so the webhook
+	 * can later reconcile the charge back to this row.
+	 */
+	public function setStripeIntentId( int $id, string $intentId ): bool {
+		return false !== $this->db->update(
+			$this->table,
+			[ 'stripe_payment_intent_id' => $intentId ],
+			[ 'id' => $id ],
+			[ '%s' ],
+			[ '%d' ]
+		);
+	}
+
+	public function findByStripeIntentId( string $intentId ): ?Payment {
+		$row = $this->db->get_row(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE stripe_payment_intent_id = %s ORDER BY id DESC LIMIT 1",
+				$intentId
+			)
+		);
+
+		return $row ? Payment::fromRow( $row ) : null;
+	}
+
 	public function updateEtransferEmail( int $id, ?string $email ): bool {
 		return false !== $this->db->update(
 			$this->table,