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/routes/v1/tax.ts

@@ -0,0 +1,15 @@
+import type { FastifyPluginAsync } from 'fastify'
+import { TaxService } from '../../services/tax.service.js'
+
+export const taxRoutes: FastifyPluginAsync = async (app) => {
+  app.get('/tax/lookup/:zip', { preHandler: [app.authenticate, app.requirePermission('pos.view')] }, async (request, reply) => {
+    const { zip } = request.params as { zip: string }
+
+    if (!/^\d{5}(-\d{4})?$/.test(zip)) {
+      return reply.status(400).send({ error: { message: 'Invalid zip code format', statusCode: 400 } })
+    }
+
+    const result = await TaxService.lookupByZip(zip)
+    return reply.send(result)
+  })
+}