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/backend/src/db/schema/inventory.ts

@@ -8,6 +8,7 @@ import {
   integer,
   numeric,
   date,
+  pgEnum,
 } from 'drizzle-orm/pg-core'
 import { locations } from './stores.js'
 
@@ -41,6 +42,8 @@ export const suppliers = pgTable('supplier', {
 // See lookups.ts for inventory_unit_status and item_condition tables.
 // Columns below use varchar referencing the lookup slug.
 
+export const taxCategoryEnum = pgEnum('tax_category', ['goods', 'service', 'exempt'])
+
 export const products = pgTable('product', {
   id: uuid('id').primaryKey().defaultRandom(),
   locationId: uuid('location_id').references(() => locations.id),
@@ -54,6 +57,7 @@ export const products = pgTable('product', {
   isSerialized: boolean('is_serialized').notNull().default(false),
   isRental: boolean('is_rental').notNull().default(false),
   isDualUseRepair: boolean('is_dual_use_repair').notNull().default(false),
+  taxCategory: taxCategoryEnum('tax_category').notNull().default('goods'),
   price: numeric('price', { precision: 10, scale: 2 }),
   minPrice: numeric('min_price', { precision: 10, scale: 2 }),
   rentalRateMonthly: numeric('rental_rate_monthly', { precision: 10, scale: 2 }),