feat: add drawer cash in/out adjustments with balance reconciliation

- New drawer_adjustment table (type: cash_in/cash_out, amount, reason)
- POST/GET /drawer/:id/adjustments endpoints
- Drawer close calculation now includes adjustments: expected = opening + sales + cash_in - cash_out
- DrawerAdjustmentSchema for input validation
- 5 new tests (44 total POS tests passing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/backend/src/db/migrations/meta/_journal.json

@@ -288,6 +288,13 @@
       "when": 1775494000000,
       "tag": "0040_app-config",
       "breakpoints": true
+    },
+    {
+      "idx": 41,
+      "version": "7",
+      "when": 1775580000000,
+      "tag": "0041_drawer-adjustments",
+      "breakpoints": true
     }
   ]
 }