test(accounting): unit tests for expense markup / disposition logic

Covers the silently-regressable money + classification bits of the re-bill
flow (the maintainer's "thin CRM test coverage" concern). Pure functions via a
new expenseService._internal export — no DB, no date-harness pitfalls:

- computeMarkupMinor: percent rounding, flat, none/null.
- resolveMarkup precedence: override > expense clause > none.
- buildExpenseInsert: bad-disposition guard, tax_treatment/status defaults,
  declined -> status+reason, markup field matches type, parked -> status.

11 tests, all green (npx jest expenseService.markup).

backend/src/services/expenseService.js

@@ -533,4 +533,6 @@ module.exports = {
   TAX_TREATMENTS,
   MARKUP_TYPES,
   PAYMENT_METHODS,
+  // exposed for unit tests
+  _internal: { computeMarkupMinor, resolveMarkup, buildExpenseInsert, transformExpense, transformInbound },
 };