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/transaction.service.ts

@@ -231,10 +231,26 @@ export const TransactionService = {
     if (!txn) throw new NotFoundError('Transaction')
     if (txn.status !== 'pending') throw new ConflictError('Transaction is not pending')
 
-    // Validate cash payment
+    // Validate cash payment (with optional nickel rounding)
     let changeGiven: string | undefined
+    let roundingAdjustment = 0
     if (input.paymentMethod === 'cash') {
-      const total = parseFloat(txn.total)
+      let total = parseFloat(txn.total)
+
+      // Apply Swedish rounding if location has cash_rounding enabled
+      if (txn.locationId) {
+        const [loc] = await db
+          .select({ cashRounding: locations.cashRounding })
+          .from(locations)
+          .where(eq(locations.id, txn.locationId))
+          .limit(1)
+        if (loc?.cashRounding) {
+          const rounded = TaxService.roundToNickel(total)
+          roundingAdjustment = Math.round((rounded - total) * 100) / 100
+          total = rounded
+        }
+      }
+
       if (!input.amountTendered || input.amountTendered < total) {
         throw new ValidationError('Amount tendered must be >= transaction total for cash payments')
       }
@@ -273,13 +289,13 @@ export const TransactionService = {
         paymentMethod: input.paymentMethod,
         amountTendered: input.amountTendered?.toString(),
         changeGiven,
+        roundingAdjustment: roundingAdjustment.toString(),
         checkNumber: input.checkNumber,
         completedAt: new Date(),
         updatedAt: new Date(),
       })
       .where(eq(transactions.id, transactionId))
       .returning()
-
     return completed
   },