Fix portal user linking: use updateResource instead of set_value for child table

set_value doesn't work for child tables like portal_users. Use
updateResource (PUT /api/resource/Customer) which handles it correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/src/routes/checkout.ts

@@ -1225,11 +1225,8 @@ export async function handlePaymentComplete(
             erpCustomer = custResult.customerName;
             // Link portal user to Customer so portal shows their orders
             try {
-              await callMethod("frappe.client.set_value", {
-                doctype: "Customer",
-                name: erpCustomer,
-                fieldname: "portal_users",
-                value: [{ user: custEmail }],
+              await updateResource("Customer", erpCustomer!, {
+                portal_users: [{ user: custEmail }],
               });
             } catch { /* non-fatal — may already be linked */ }
           } catch { /* non-fatal */ }