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/services/tax.service.ts

@@ -44,6 +44,14 @@ export const TaxService = {
     return Math.round(amount * rate * 100) / 100
   },
 
+  /**
+   * Swedish rounding: round to nearest $0.05 for cash payments.
+   * Only affects the final total — tax and line items stay exact.
+   */
+  roundToNickel(amount: number): number {
+    return Math.round(amount * 20) / 20
+  },
+
   /**
    * Map repair line item types to tax categories:
    * - "part"      → goods (taxable)