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/db/src/schema.ts

@@ -26,6 +26,20 @@ export const staffRoleEnum = pgEnum("staff_role", [
   "manager",
 ]);
 
+export const invoiceStatusEnum = pgEnum("invoice_status", [
+  "draft",
+  "pending",
+  "paid",
+  "void",
+]);
+
+export const paymentMethodEnum = pgEnum("payment_method", [
+  "cash",
+  "card",
+  "check",
+  "other",
+]);
+
 // ─── Tables ───────────────────────────────────────────────────────────────────
 
 export const clients = pgTable("clients", {
@@ -101,3 +115,35 @@ export const appointments = pgTable("appointments", {
   createdAt: timestamp("created_at").notNull().defaultNow(),
   updatedAt: timestamp("updated_at").notNull().defaultNow(),
 });
+
+export const invoices = pgTable("invoices", {
+  id: uuid("id").primaryKey().defaultRandom(),
+  appointmentId: uuid("appointment_id").references(() => appointments.id, {
+    onDelete: "restrict",
+  }),
+  clientId: uuid("client_id")
+    .notNull()
+    .references(() => clients.id, { onDelete: "restrict" }),
+  subtotalCents: integer("subtotal_cents").notNull(),
+  taxCents: integer("tax_cents").notNull().default(0),
+  tipCents: integer("tip_cents").notNull().default(0),
+  totalCents: integer("total_cents").notNull(),
+  status: invoiceStatusEnum("status").notNull().default("draft"),
+  paymentMethod: paymentMethodEnum("payment_method"),
+  paidAt: timestamp("paid_at"),
+  notes: text("notes"),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+  updatedAt: timestamp("updated_at").notNull().defaultNow(),
+});
+
+export const invoiceLineItems = pgTable("invoice_line_items", {
+  id: uuid("id").primaryKey().defaultRandom(),
+  invoiceId: uuid("invoice_id")
+    .notNull()
+    .references(() => invoices.id, { onDelete: "cascade" }),
+  description: text("description").notNull(),
+  quantity: integer("quantity").notNull().default(1),
+  unitPriceCents: integer("unit_price_cents").notNull(),
+  totalCents: integer("total_cents").notNull(),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+});