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>

apps/api/src/index.ts

@@ -7,6 +7,7 @@ import { petsRouter } from "./routes/pets.js";
 import { servicesRouter } from "./routes/services.js";
 import { appointmentsRouter } from "./routes/appointments.js";
 import { staffRouter } from "./routes/staff.js";
+import { invoicesRouter } from "./routes/invoices.js";
 import { authMiddleware } from "./middleware/auth.js";
 
 const app = new Hono();
@@ -33,6 +34,7 @@ api.route("/pets", petsRouter);
 api.route("/services", servicesRouter);
 api.route("/appointments", appointmentsRouter);
 api.route("/staff", staffRouter);
+api.route("/invoices", invoicesRouter);
 
 const port = Number(process.env.PORT ?? 3000);
 console.log(`API server listening on port ${port}`);