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 <noreply@anthropic.com>

tests/Unit/Payment/PaymentRepositoryTest.php

@@ -98,6 +98,42 @@ class PaymentRepositoryTest extends TestCase
         self::assertCount(0, $this->repo->findPaidBetween('2026-06-01 00:00:00', '2026-07-01 00:00:00'));
     }
 
+    public function testSetStripeIntentIdUpdatesRow(): void
+    {
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with(
+                'wp_us_payments',
+                ['stripe_payment_intent_id' => 'pi_123'],
+                ['id' => 50],
+                ['%s'],
+                ['%d']
+            )
+            ->andReturn(1);
+
+        self::assertTrue($this->repo->setStripeIntentId(50, 'pi_123'));
+    }
+
+    public function testFindByStripeIntentIdReturnsPayment(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/stripe_payment_intent_id = %s/'), 'pi_123')
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_row')->andReturn($this->row());
+
+        self::assertInstanceOf(Payment::class, $this->repo->findByStripeIntentId('pi_123'));
+    }
+
+    public function testFindByStripeIntentIdReturnsNullWhenMissing(): void
+    {
+        $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
+        $this->db->shouldReceive('get_row')->andReturn(null);
+
+        self::assertNull($this->repo->findByStripeIntentId('pi_missing'));
+    }
+
     public function testFindByRegistrationReturnsPayment(): void
     {
         $this->db->shouldReceive('prepare')