Gate payment gateway behind Pro and Business plans.

Free accounts keep cash sales only; connecting a gateway and card/MoMo
checkout require a paid plan, with settings upsell copy for free users.

app/Services/Pos/SubscriptionService.php

@@ -69,6 +69,12 @@ class SubscriptionService
         return $this->hasPaidPlan($user);
     }
 
+    /** Own payment gateway (Paystack / Flutterwave / Hubtel) requires Pro or Business. */
+    public function canUsePaymentGateway(User $user): bool
+    {
+        return $this->hasPaidPlan($user);
+    }
+
     public function productCount(string $ownerRef, bool $restaurant): int
     {
         if ($restaurant) {