Add Stripe subscription integration with pricing plans

prisma/schema.prisma

@@ -19,13 +19,26 @@ model User {
   emailVerified DateTime?
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
-  
+
+  // Stripe subscription fields
+  stripeCustomerId       String?   @unique
+  stripeSubscriptionId   String?   @unique
+  stripePriceId          String?
+  stripeCurrentPeriodEnd DateTime?
+  plan                   Plan      @default(FREE)
+
   qrCodes       QRCode[]
   integrations  Integration[]
   accounts      Account[]
   sessions      Session[]
 }
 
+enum Plan {
+  FREE
+  PRO
+  BUSINESS
+}
+
 model Account {
   id                String  @id @default(cuid())
   userId            String