Fix Stripe checkout: wrong column name stripe_checkout_session → stripe_session_id

DB column is stripe_session_id but code was writing to stripe_checkout_session,
causing a 500 on checkout and breaking payment status checks.

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

api/create-checkout-session.php

@@ -103,7 +103,7 @@ try {
     
     // Store checkout session ID
     db()->update('orders',
-        ['stripe_checkout_session' => $session['id']],
+        ['stripe_session_id' => $session['id']],
         'order_id = :id',
         ['id' => $orderId]
     );