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/migrations/0039_cash-rounding.sql

@@ -0,0 +1,3 @@
+-- Cash rounding: location setting + transaction adjustment tracking
+ALTER TABLE "location" ADD COLUMN "cash_rounding" boolean NOT NULL DEFAULT false;
+ALTER TABLE "transaction" ADD COLUMN "rounding_adjustment" numeric(10, 2) NOT NULL DEFAULT '0';