feat: basic POS & invoicing (closes groombook/groombook#5) (#26)

- Add invoice_status and payment_method enums to schema
- Add invoices table: appointmentId, clientId, subtotal/tax/tip/total cents,
  status (draft/pending/paid/void), paymentMethod, paidAt, notes
- Add invoice_line_items table: invoiceId, description, qty, unitPrice, total
- Migration 0002_invoices.sql with FK constraints and journal entry
- POST /api/invoices — create invoice with line items
- POST /api/invoices/from-appointment/:id — one-click invoice from appointment,
  pre-populated with service name and price; returns 409 if already invoiced
- GET /api/invoices — list with optional ?status/clientId/appointmentId filters
- GET /api/invoices/:id — invoice with line items
- PATCH /api/invoices/:id — update status, payment method, tip, notes; auto-sets
  paidAt when marking paid; blocks edits on voided invoices
- Add Invoice/InvoiceLineItem types to @groombook/types
- InvoicesPage: list view with status filter, create from appointment modal,
  detail modal with tip input, payment method selector, Mark as Paid/Void actions
- Add Invoices nav link in App.tsx

Co-authored-by: Groom Book CTO <cto@groombook.app>
Co-authored-by: Paperclip <noreply@paperclip.ing>

packages/types/src/index.ts

@@ -69,6 +69,36 @@ export interface Appointment {
   updatedAt: string;
 }
 
+export type InvoiceStatus = "draft" | "pending" | "paid" | "void";
+export type PaymentMethod = "cash" | "card" | "check" | "other";
+
+export interface InvoiceLineItem {
+  id: string;
+  invoiceId: string;
+  description: string;
+  quantity: number;
+  unitPriceCents: number;
+  totalCents: number;
+  createdAt: string;
+}
+
+export interface Invoice {
+  id: string;
+  appointmentId: string | null;
+  clientId: string;
+  subtotalCents: number;
+  taxCents: number;
+  tipCents: number;
+  totalCents: number;
+  status: InvoiceStatus;
+  paymentMethod: PaymentMethod | null;
+  paidAt: string | null;
+  notes: string | null;
+  createdAt: string;
+  updatedAt: string;
+  lineItems?: InvoiceLineItem[];
+}
+
 // Paginated list response
 export interface PaginatedList<T> {
   items: T[];