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/db/schema/pos.ts

@@ -104,6 +104,7 @@ export const transactions = pgTable('transaction', {
   changeGiven: numeric('change_given', { precision: 10, scale: 2 }),
   checkNumber: varchar('check_number', { length: 50 }),
   stripePaymentIntentId: varchar('stripe_payment_intent_id', { length: 255 }),
+  roundingAdjustment: numeric('rounding_adjustment', { precision: 10, scale: 2 }).notNull().default('0'),
   taxExempt: boolean('tax_exempt').notNull().default(false),
   taxExemptReason: text('tax_exempt_reason'),
   processedBy: uuid('processed_by')