feat: add core POS module — transactions, discounts, drawer, tax

Phase 3a backend API for point-of-sale. Includes:

Schema (packages/backend/src/db/schema/pos.ts):
- pgEnums: transaction_type, transaction_status, payment_method,
  discount_type, discount_applies_to, drawer_status
- Tables: transaction, transaction_line_item, discount,
  discount_audit, drawer_session
- Transaction links to accounts, repair_tickets, repair_batches
- Line items link to products and inventory_units

Tax system:
- tax_rate + service_tax_rate columns on location
- tax_category enum (goods/service/exempt) on product
- Tax resolves per line item: goods→tax_rate, service→service_tax_rate,
  exempt→0. Repair line items map: part→goods, labor→service
- GET /tax/lookup/:zip stubbed for future API integration (TAX_API_KEY)

Services (export const pattern, matching existing codebase):
- TransactionService: create, addLineItem, removeLineItem, applyDiscount,
  recalculateTotals, complete (decrements inventory), void, getReceipt
- DiscountService: CRUD + listAll for dropdowns
- DrawerService: open/close with expected balance + over/short calc
- TaxService: getRateForLocation (by tax category), calculateTax

Routes:
- POST/GET /transactions, GET /transactions/:id, GET /transactions/:id/receipt
- POST /transactions/:id/line-items, DELETE /transactions/:id/line-items/:id
- POST /transactions/:id/discounts, /complete, /void
- POST /drawer/open, POST /drawer/:id/close, GET /drawer/current, GET /drawer
- CRUD /discounts + GET /discounts/all
- GET /products/lookup/upc/:upc (barcode scanner support)

All routes gated by pos.view/pos.edit/pos.admin + withModule('pos').
POS module already seeded in migration 0026.

Still needed: bun install, drizzle-kit generate + migrate, tests, lint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared/src/schemas/index.ts

@@ -163,3 +163,31 @@ export type {
   LessonPlanTemplateUpdateInput,
   TemplateInstantiateInput,
 } from './lessons.schema.js'
+
+export {
+  TransactionType,
+  TransactionStatus,
+  PaymentMethod,
+  DiscountType,
+  DiscountAppliesTo,
+  DrawerStatus,
+  TaxCategory,
+  TransactionCreateSchema,
+  TransactionLineItemCreateSchema,
+  ApplyDiscountSchema,
+  CompleteTransactionSchema,
+  DiscountCreateSchema,
+  DiscountUpdateSchema,
+  DrawerOpenSchema,
+  DrawerCloseSchema,
+} from './pos.schema.js'
+export type {
+  TransactionCreateInput,
+  TransactionLineItemCreateInput,
+  ApplyDiscountInput,
+  CompleteTransactionInput,
+  DiscountCreateInput,
+  DiscountUpdateInput,
+  DrawerOpenInput,
+  DrawerCloseInput,
+} from './pos.schema.js'

packages/shared/src/schemas/pos.schema.ts

@@ -0,0 +1,114 @@
+import { z } from 'zod'
+
+/** Coerce empty strings to undefined — solves HTML form inputs sending '' for blank optional fields */
+function opt<T extends z.ZodTypeAny>(schema: T) {
+  return z.preprocess((v) => (v === '' ? undefined : v), schema.optional())
+}
+
+// --- Enum values (must match pgEnum definitions in pos.ts) ---
+
+export const TransactionType = z.enum([
+  'sale',
+  'repair_payment',
+  'rental_deposit',
+  'account_payment',
+  'refund',
+])
+export type TransactionType = z.infer<typeof TransactionType>
+
+export const TransactionStatus = z.enum(['pending', 'completed', 'voided', 'refunded'])
+export type TransactionStatus = z.infer<typeof TransactionStatus>
+
+export const PaymentMethod = z.enum([
+  'cash',
+  'card_present',
+  'card_keyed',
+  'check',
+  'account_charge',
+])
+export type PaymentMethod = z.infer<typeof PaymentMethod>
+
+export const DiscountType = z.enum(['percent', 'fixed'])
+export type DiscountType = z.infer<typeof DiscountType>
+
+export const DiscountAppliesTo = z.enum(['order', 'line_item', 'category'])
+export type DiscountAppliesTo = z.infer<typeof DiscountAppliesTo>
+
+export const DrawerStatus = z.enum(['open', 'closed'])
+export type DrawerStatus = z.infer<typeof DrawerStatus>
+
+export const TaxCategory = z.enum(['goods', 'service', 'exempt'])
+export type TaxCategory = z.infer<typeof TaxCategory>
+
+// --- Transaction schemas ---
+
+export const TransactionCreateSchema = z.object({
+  transactionType: TransactionType,
+  locationId: opt(z.string().uuid()),
+  accountId: opt(z.string().uuid()),
+  repairTicketId: opt(z.string().uuid()),
+  repairBatchId: opt(z.string().uuid()),
+  notes: opt(z.string()),
+  taxExempt: z.boolean().default(false),
+  taxExemptReason: opt(z.string()),
+})
+export type TransactionCreateInput = z.infer<typeof TransactionCreateSchema>
+
+export const TransactionLineItemCreateSchema = z.object({
+  productId: opt(z.string().uuid()),
+  inventoryUnitId: opt(z.string().uuid()),
+  description: z.string().min(1).max(255),
+  qty: z.number().int().min(1).default(1),
+  unitPrice: z.coerce.number().min(0),
+})
+export type TransactionLineItemCreateInput = z.infer<typeof TransactionLineItemCreateSchema>
+
+export const ApplyDiscountSchema = z.object({
+  discountId: opt(z.string().uuid()),
+  amount: z.coerce.number().min(0),
+  reason: z.string().min(1),
+  lineItemId: opt(z.string().uuid()),
+})
+export type ApplyDiscountInput = z.infer<typeof ApplyDiscountSchema>
+
+export const CompleteTransactionSchema = z.object({
+  paymentMethod: PaymentMethod,
+  amountTendered: z.coerce.number().min(0).optional(),
+  checkNumber: opt(z.string().max(50)),
+})
+export type CompleteTransactionInput = z.infer<typeof CompleteTransactionSchema>
+
+// --- Discount schemas ---
+
+export const DiscountCreateSchema = z.object({
+  name: z.string().min(1).max(255),
+  discountType: DiscountType,
+  discountValue: z.coerce.number().min(0),
+  appliesTo: DiscountAppliesTo.default('line_item'),
+  locationId: opt(z.string().uuid()),
+  requiresApprovalAbove: z.coerce.number().min(0).optional(),
+  isActive: z.boolean().default(true),
+  validFrom: opt(z.string()),
+  validUntil: opt(z.string()),
+})
+export type DiscountCreateInput = z.infer<typeof DiscountCreateSchema>
+
+export const DiscountUpdateSchema = DiscountCreateSchema.partial()
+export type DiscountUpdateInput = z.infer<typeof DiscountUpdateSchema>
+
+// --- Drawer schemas ---
+
+export const DrawerOpenSchema = z.object({
+  locationId: opt(z.string().uuid()),
+  openingBalance: z.coerce.number().min(0),
+})
+export type DrawerOpenInput = z.infer<typeof DrawerOpenSchema>
+
+export const DrawerCloseSchema = z.object({
+  closingBalance: z.coerce.number().min(0),
+  denominations: z
+    .record(z.string(), z.number())
+    .optional(),
+  notes: opt(z.string()),
+})
+export type DrawerCloseInput = z.infer<typeof DrawerCloseSchema>