feat(gro-609): add refund handling and payment stats to admin

- Add stripePaymentIntentId to Invoice schema and types
- Add POST /api/invoices/:id/refund endpoint (Stripe placeholder)
- Add GET /api/invoices/stats/summary for payment analytics
- Add refund button + dialog (full/partial) to InvoiceDetailModal
- Add payment stats cards to Invoices page (revenue, outstanding, refunds, method breakdown)

Ref: GRO-609
Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/types/src/index.ts

@@ -152,6 +152,9 @@ export interface Invoice {
   status: InvoiceStatus;
   paymentMethod: PaymentMethod | null;
   paidAt: string | null;
+  stripePaymentIntentId: string | null;
+  stripeRefundId: string | null;
+  paymentFailureReason: string | null;
   notes: string | null;
   createdAt: string;
   updatedAt: string;