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/meta/_journal.json

@@ -267,6 +267,20 @@
       "when": 1774970000000,
       "tag": "0037_rate_cycles",
       "breakpoints": true
+    },
+    {
+      "idx": 38,
+      "version": "7",
+      "when": 1775321562910,
+      "tag": "0038_pos-core",
+      "breakpoints": true
+    },
+    {
+      "idx": 39,
+      "version": "7",
+      "when": 1775408000000,
+      "tag": "0039_cash-rounding",
+      "breakpoints": true
     }
   ]
 }