fix: lazy-initialize Stripe client to prevent build-time errors

Stripe SDK was being initialized at module load time, which failed
during Next.js build when STRIPE_SECRET_KEY wasn't set in the build
environment. Changed to lazy initialization pattern with proxy object
that creates the client on first access.

Fixes: Error: Neither apiKey nor config.authenticator provided

src/lib/stripe-billing.ts

@@ -3,10 +3,33 @@
 
 import Stripe from "stripe";
 
-// Initialize Stripe
-const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, {
+// Lazy-initialize Stripe client to avoid build-time errors when env vars aren't set
+let _stripe: Stripe | null = null;
+
+function getStripeClient(): Stripe {
+  if (!_stripe) {
+    if (!process.env.STRIPE_SECRET_KEY) {
+      throw new Error("STRIPE_SECRET_KEY environment variable is not set");
+    }
+    _stripe = new Stripe(process.env.STRIPE_SECRET_KEY, {
       apiVersion: "2026-05-27.dahlia",
     });
+  }
+  return _stripe;
+}
+
+// Export stripe proxy that lazily initializes
+export const stripe = {
+  get customers() { return getStripeClient().customers; },
+  get subscriptions() { return getStripeClient().subscriptions; },
+  get invoices() { return getStripeClient().invoices; },
+  get checkout() { return getStripeClient().checkout; },
+  get billingPortal() { return getStripeClient().billingPortal; },
+  get paymentMethods() { return getStripeClient().paymentMethods; },
+  get paymentIntents() { return getStripeClient().paymentIntents; },
+  get refunds() { return getStripeClient().refunds; },
+  get webhooks() { return getStripeClient().webhooks; },
+};
 
 // Plan tier configurations
 export const PLANS = {
@@ -667,8 +690,6 @@ export async function getUsageSummary(subscriptionItemId: string) {
 // EXPORT STRIPE INSTANCE AND HELPERS
 // ============================================
 
-export { stripe };
-
 export async function getSubscription(subscriptionId: string) {
   return await stripe.subscriptions.retrieve(subscriptionId);
 }