test(crm): dunning fee math, VAT-toggle gating + invoice immutability

Covers the tax-sensitive bits the dunning rework added (previously untested):
flat vs percent fee, the VAT toggle applying the org rate AND no-op'ing when
the org has no VAT rate, per-reminder accumulation (2nd=1x / 3rd=2x), the
invoice total staying immutable while the fee is tracked, and the 3-reminder
cap. Exports the fee resolvers + applyReminder for testing; PDF render stubbed
(flaky in CI, verified manually). 6/6 pass.

backend/src/services/invoiceService.js

@@ -3528,6 +3528,10 @@ module.exports = {
   validateInstallmentPlanInput,
   sendInvoice,
   sendReminder,
+  applyReminder,
+  resolveLateFeeNetMinor,
+  resolveLateFeeVatRate,
+  resolvePerReminderFeeMinor,
   markPaid,
   cancelInvoice,
   releaseForDelivery,