Enable Plaid balance verification for ACH payments

Adds financial_connections with balances permission and prefetch
to Stripe Checkout sessions when payment_method is ACH. This uses
Plaid to verify the customer's bank account balance before accepting
payment, reducing ACH return risk.

- permissions: ["payment_method", "balances"]
- prefetch: ["balances"]
- verification_method: "instant"

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

api/src/routes/checkout.ts

@@ -979,6 +979,18 @@ router.post("/api/v1/checkout/create-session", async (req, res) => {
         ...(erpnextCustomer ? { erpnext_customer: erpnextCustomer } : {}),
       },
       payment_intent_data: paymentIntentData,
+      // ACH via Plaid: verify account balance before accepting payment
+      ...(payment_method === "ach" ? {
+        payment_method_options: {
+          us_bank_account: {
+            financial_connections: {
+              permissions: ["payment_method", "balances"],
+              prefetch: ["balances"],
+            },
+            verification_method: "instant",
+          },
+        },
+      } : {}),
     });
 
     // (Sales Order already created above, before gateway split)