feat: add cash rounding, POS test suite, and fix test harness port cleanup

- Add Swedish rounding (nearest nickel) for cash payments at locations with cash_rounding enabled
- Add rounding_adjustment column to transactions, cash_rounding to locations
- Add POS schema to database plugin for relational query support
- Complete/void routes now return full transaction with line items via getById
- Test harness killPort falls back to fuser when lsof unavailable (fixes stale process bug)
- Add 35-test POS API suite covering discounts, drawer, transactions, tax, rounding, e2e flow
- Add unit tests for tax service and POS Zod schemas

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

packages/backend/src/plugins/database.ts

@@ -5,8 +5,9 @@ import * as storeSchema from '../db/schema/stores.js'
 import * as userSchema from '../db/schema/users.js'
 import * as accountSchema from '../db/schema/accounts.js'
 import * as inventorySchema from '../db/schema/inventory.js'
+import * as posSchema from '../db/schema/pos.js'
 
-const schema = { ...storeSchema, ...userSchema, ...accountSchema, ...inventorySchema }
+const schema = { ...storeSchema, ...userSchema, ...accountSchema, ...inventorySchema, ...posSchema }
 
 declare module 'fastify' {
   interface FastifyInstance {