Fix monthly actuals: allow negative values and fix save/reconcile error

- Remove min={0} from NumberInput to allow negative actuals (refunds/corrections)
- Fix post() in journal-entries service: use id param directly instead of
  RETURNING result which returns [rows, count] in TypeORM QueryRunner
- Handle negative amounts in saveActuals(): negative expense credits the
  expense account, negative income debits the income account

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/monthly-actuals/MonthlyActualsPage.tsx

@@ -203,7 +203,7 @@ export function MonthlyActualsPage() {
                 size="xs"
                 hideControls
                 decimalScale={2}
-                min={0}
+                allowNegative
                 styles={{ input: { textAlign: 'right', fontFamily: 'monospace' } }}
               />
             </Table.Td>