feat(crm): toggle for VAT on late fees (jurisdiction-dependent)

Mahngebühr VAT differs by country (CH: liable; DE/AT: not), so it's now a
toggle (crm_invoices_late_fee_vat_enabled, seeded into migration 143 in place
since it isn't deployed yet — no compensation migration). When on, VAT is
added on top of the net fee at the org's default rate
(business_profile.vat_rate_default). Gated so it's a NO-OP when the org doesn't
charge VAT (default rate 0/unset) — i.e. enabling the toggle on a non-VAT org
adds nothing, as required. Settings UI: a self-documenting checkbox.

The fee is treated as net + VAT-on-top; the tax-report VAT breakdown for the
fee is part of the deferred dunning-document rework. tsc 0, build green,
9/9 workflow tests.

backend/migrations/core/143_seed_late_fee_type.js

@@ -19,6 +19,10 @@ exports.up = async function (knex) {
   const seeds = [
     { setting_key: 'crm_invoices_late_fee_type', setting_value: JSON.stringify('flat'), setting_type: 'crm' },
     { setting_key: 'crm_invoices_late_fee_percent', setting_value: JSON.stringify(0), setting_type: 'crm' },
+    // VAT on the late fee is jurisdiction-dependent (CH: yes; DE/AT: no), so it's
+    // a toggle. Default OFF (preserve current no-VAT behaviour). No-op anyway
+    // when the org doesn't charge VAT (business_profile.vat_rate_default = 0).
+    { setting_key: 'crm_invoices_late_fee_vat_enabled', setting_value: JSON.stringify(false), setting_type: 'crm' },
   ];
   for (const s of seeds) {
     const exists = await knex('app_settings').where({ setting_key: s.setting_key }).first();
@@ -29,6 +33,6 @@ exports.up = async function (knex) {
 exports.down = async function (knex) {
   if (!(await knex.schema.hasTable('app_settings'))) return;
   await knex('app_settings')
-    .whereIn('setting_key', ['crm_invoices_late_fee_type', 'crm_invoices_late_fee_percent'])
+    .whereIn('setting_key', ['crm_invoices_late_fee_type', 'crm_invoices_late_fee_percent', 'crm_invoices_late_fee_vat_enabled'])
     .del();
 };