feat(GRO-609): admin refund handling and payment stats dashboard

- Add refund button to invoice detail modal (paid Stripe invoices only)
- Full refund (default) and partial refund via confirmation dialog
- Call POST /api/invoices/:id/refund on confirmation
- Display Stripe payment info (card last4, brand, payment status)
- Show refund status when invoice has been refunded
- Add payment stats section to InvoicesPage (revenue, outstanding, refunds, method breakdown)
- Add GET /api/invoices/:id/stripe-payment endpoint for card details
- Add GET /api/invoices/stats endpoint for monthly payment stats
- Extend Invoice type with Stripe fields (stripePaymentIntentId, stripeRefundId)

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

packages/types/src/index.ts

@@ -153,10 +153,38 @@ export interface Invoice {
   notes: string | null;
   createdAt: string;
   updatedAt: string;
+  stripePaymentIntentId?: string | null;
+  stripeRefundId?: string | null;
+  paymentFailureReason?: string | null;
   lineItems?: InvoiceLineItem[];
   tipSplits?: InvoiceTipSplit[];
 }
 
+export interface StripePaymentInfo {
+  paymentIntentId: string;
+  amountPaidCents: number;
+  status: string;
+  cardLast4: string | null;
+  cardBrand: string | null;
+  refundId: string | null;
+  refundStatus: string | null;
+}
+
+export interface PaymentMethodBreakdown {
+  paymentMethod: PaymentMethod;
+  count: number;
+  totalCents: number;
+}
+
+export interface PaymentStats {
+  revenueCents: number;
+  outstandingCents: number;
+  refundsCents: number;
+  revenueCount: number;
+  refundCount: number;
+  paymentMethodBreakdown: PaymentMethodBreakdown[];
+}
+
 // ─── Impersonation ──────────────────────────────────────────────────────────
 
 export type ImpersonationSessionStatus = "active" | "ended" | "expired";