fix: code review fixes + unit/API tests for repair-POS integration

Code review fixes:
- Wrap createFromRepairTicket() in DB transaction for atomicity
- Wrap complete() inventory + status updates in DB transaction
- Repair ticket status update now atomic with transaction completion
- Add Zod validation on from-repair route body
- Fix requiresDiscountOverride: threshold and manual_discount are independent checks
- Order discount distributes proportionally across line items (not first-only)
- Extract shared receipt calculations into useReceiptData/useBarcode hooks
- Add error handling for barcode generation

Tests:
- Unit: consumable tax category mapping, exempt rate short-circuit
- API: ready-for-pickup listing + search, from-repair transaction creation,
  consumable exclusion from line items, tax rate verification (labor=service,
  part=goods), duplicate prevention, ticket auto-pickup on payment completion,
  isConsumable product filter

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

packages/backend/__tests__/services/tax-consumable.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from 'bun:test'
+import { TaxService } from '../../src/services/tax.service.js'
+
+describe('TaxService.repairItemTypeToTaxCategory — consumable', () => {
+  it('maps consumable to exempt', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('consumable')).toBe('exempt')
+  })
+
+  it('maps labor to service', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('labor')).toBe('service')
+  })
+
+  it('maps part to goods', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('part')).toBe('goods')
+  })
+
+  it('maps flat_rate to goods', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('flat_rate')).toBe('goods')
+  })
+
+  it('maps misc to goods', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('misc')).toBe('goods')
+  })
+
+  it('maps unknown type to goods (default)', () => {
+    expect(TaxService.repairItemTypeToTaxCategory('anything_else')).toBe('goods')
+  })
+})
+
+describe('TaxService.getRateForLocation — exempt category', () => {
+  it('returns 0 for exempt tax category without DB call', async () => {
+    // Passing a fake DB and fake locationId — should short-circuit and return 0
+    const fakeDb = {} as any
+    const rate = await TaxService.getRateForLocation(fakeDb, 'fake-id', 'exempt')
+    expect(rate).toBe(0)
+  })
+})