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/drawer.service.ts

@@ -32,8 +32,12 @@ export const DrawerService = {
     if (session.status === 'closed') throw new ConflictError('Drawer session is already closed')
 
     // Calculate expected balance from cash transactions in this drawer session
-    const [cashTotal] = await db
-      .select({ total: sum(transactions.total) })
+    // Net cash kept = total + rounding_adjustment (change is already accounted for)
+    const [cashTotals] = await db
+      .select({
+        total: sum(transactions.total),
+        rounding: sum(transactions.roundingAdjustment),
+      })
       .from(transactions)
       .where(
         and(
@@ -43,7 +47,7 @@ export const DrawerService = {
         )
       )
 
-    const cashIn = parseFloat(cashTotal?.total ?? '0')
+    const cashIn = parseFloat(cashTotals?.total ?? '0') + parseFloat(cashTotals?.rounding ?? '0')
     const openingBalance = parseFloat(session.openingBalance)
     const expectedBalance = openingBalance + cashIn
     const closingBalance = input.closingBalance