fix: attach payment method to customer before creating subscription

Stripe requires payment methods to be attached to a customer before they
can be set as default_payment_method on a subscription. The
createSubscription() method was skipping this step, causing 500 errors
on checkout with: "The customer does not have a payment method with the
ID pm_xxx".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/subscriptions/external/stripe/stripe.client.ts

@@ -75,6 +75,14 @@ export class StripeClient {
     try {
       logger.info('Creating Stripe subscription', { customerId, priceId, paymentMethodId });
 
+      // Attach payment method to customer before creating subscription
+      if (paymentMethodId) {
+        await this.stripe.paymentMethods.attach(paymentMethodId, {
+          customer: customerId,
+        });
+        logger.info('Payment method attached to customer', { customerId, paymentMethodId });
+      }
+
       const subscriptionParams: Stripe.SubscriptionCreateParams = {
         customer: customerId,
         items: [{ price: priceId }],