fix(GRO-818): refund button for all paid invoices, inline cardLast4, manual refund for non-Stripe

- Backend refund endpoint: allow refunds on paid invoices without stripePaymentIntentId (manual refund path)
- Backend GET /invoices/🆔 inline fetch cardLast4 + paymentStatus from Stripe when stripePaymentIntentId present
- Frontend: show Refund button on all paid invoices for managers (not just Stripe-backed ones)
- Seed: add stripePaymentIntentId (pi_test_*) to ~20% of paid invoices for Stripe-path testing

cc @cpfarhood

apps/web/src/pages/Invoices.tsx

@@ -487,7 +487,7 @@ const [showRefundDialog, setShowRefundDialog] = useState(false);
             </div>
           )}
           <div style={{ display: "flex", gap: "0.5rem", justifyContent: "flex-end" }}>
-            {invoice.status === "paid" && invoice.stripePaymentIntentId && !invoice.stripeRefundId && isManager && (
+            {invoice.status === "paid" && !invoice.stripeRefundId && isManager && (
               <button onClick={() => setShowRefundDialog(true)} style={{ ...btnStyle, color: "#fff", backgroundColor: "#7c3aed", borderColor: "#7c3aed" }}>
                 Refund
               </button>