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/routes/v1/transactions.ts

@@ -78,13 +78,15 @@ export const transactionRoutes: FastifyPluginAsync = async (app) => {
     if (!parsed.success) {
       return reply.status(400).send({ error: { message: 'Validation failed', details: parsed.error.flatten(), statusCode: 400 } })
     }
-    const txn = await TransactionService.complete(app.db, id, parsed.data)
+    await TransactionService.complete(app.db, id, parsed.data)
+    const txn = await TransactionService.getById(app.db, id)
     return reply.send(txn)
   })
 
   app.post('/transactions/:id/void', { preHandler: [app.authenticate, app.requirePermission('pos.admin')] }, async (request, reply) => {
     const { id } = request.params as { id: string }
-    const txn = await TransactionService.void(app.db, id, request.user.id)
+    await TransactionService.void(app.db, id, request.user.id)
+    const txn = await TransactionService.getById(app.db, id)
     return reply.send(txn)
   })
 }