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/services/tax.service.ts

@@ -0,0 +1,78 @@
+import { eq } from 'drizzle-orm'
+import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
+import { locations } from '../db/schema/stores.js'
+import { AppError } from '../lib/errors.js'
+
+export type TaxCategory = 'goods' | 'service' | 'exempt'
+
+export const TaxService = {
+  /**
+   * Get the tax rate for a location, resolved by tax category:
+   * - "goods"   → location.taxRate (default)
+   * - "service" → location.serviceTaxRate, falls back to taxRate
+   * - "exempt"  → 0
+   *
+   * Returns 0 with no warning for exempt items.
+   * Returns 0 with warning if no rate is configured on the location.
+   */
+  async getRateForLocation(
+    db: PostgresJsDatabase<any>,
+    locationId: string,
+    taxCategory: TaxCategory = 'goods',
+  ): Promise<number> {
+    if (taxCategory === 'exempt') return 0
+
+    const [location] = await db
+      .select({ taxRate: locations.taxRate, serviceTaxRate: locations.serviceTaxRate })
+      .from(locations)
+      .where(eq(locations.id, locationId))
+      .limit(1)
+
+    if (!location) return 0
+
+    if (taxCategory === 'service') {
+      // Use service rate if set, otherwise fall back to goods rate
+      const rate = location.serviceTaxRate ?? location.taxRate
+      return rate ? parseFloat(rate) : 0
+    }
+
+    // Default: goods rate
+    return location.taxRate ? parseFloat(location.taxRate) : 0
+  },
+
+  calculateTax(amount: number, rate: number): number {
+    return Math.round(amount * rate * 100) / 100
+  },
+
+  /**
+   * Map repair line item types to tax categories:
+   * - "part"      → goods (taxable)
+   * - "labor"     → service (may be taxed differently)
+   * - "flat_rate" → goods (conservative — includes parts)
+   * - "misc"      → goods (default)
+   */
+  repairItemTypeToTaxCategory(itemType: string): TaxCategory {
+    switch (itemType) {
+      case 'labor':
+        return 'service'
+      case 'part':
+      case 'flat_rate':
+      case 'misc':
+      default:
+        return 'goods'
+    }
+  },
+
+  // TODO: Integrate with a real tax rate API (TaxJar, Avalara, etc.)
+  // Set TAX_API_KEY env var when ready.
+  async lookupByZip(
+    _zip: string,
+  ): Promise<{ zip: string; rate: number; state_rate: number; county_rate: number; city_rate: number }> {
+    if (!process.env.TAX_API_KEY) {
+      throw new AppError('Tax rate lookup is not configured. Set TAX_API_KEY to enable automatic lookup.', 501)
+    }
+
+    // Placeholder — replace with real API call
+    throw new AppError('Tax rate lookup not yet implemented', 501)
+  },
+}