feat(crm): 3-reminder dunning + flat/percent Mahngebühr on 2nd & 3rd + AGB notice

- Late fee can now be a FLAT amount OR a PERCENTAGE of the invoice gross
  (crm_invoices_late_fee_type/_percent, migration 143; defaults preserve the
  current flat behaviour).
- Fee is charged from the 2nd reminder onward and accumulates per fee-bearing
  reminder (2nd = 1×, 3rd = 2×), computed from the level so re-applying a level
  never stacks. New resolvePerReminderFeeMinor() shared by applyReminder + the
  payment-check fee preview.
- Reminder ladder extended to 3 levels (caps raised in sendReminder +
  recordPaymentCheckAction); the built-in dunning flow now loops 3× (seed v3,
  re-seeds the disabled built-in on boot).
- Settings UI: flat/percent toggle + percent field, and a prominent AGB
  callout — a late fee is only enforceable if the concrete amount is stated in
  the terms (Mara's wording), 'verify with your Treuhänder'. en + native de.

The fee math is examples-only / Treuhänder-verify; issued invoices stay
immutable (the fee is tracked in late_fee_amount_minor, not folded into the
original total). Tests 17/17, tsc 0, build green.

backend/migrations/core/143_seed_late_fee_type.js

@@ -0,0 +1,34 @@
+/**
+ * Migration 143: late-fee (Mahngebühr) type — flat amount OR percentage.
+ *
+ * Extends the existing flat `crm_invoices_late_fee_minor` with a type switch so
+ * the dunning fee can be a percentage of the invoice gross instead of a fixed
+ * amount. The fee is charged from the 2nd reminder onwards (the 1st is
+ * fee-free), accumulating per fee-bearing reminder (2nd = 1×, 3rd = 2×).
+ *
+ * Seeds conservative defaults that PRESERVE current behaviour: type='flat'
+ * (so the existing flat fee keeps applying) and percent=0. Idempotent —
+ * only inserts keys that don't already exist, never clobbers an admin value.
+ *
+ * ⚠️ A late fee is only legally enforceable if the concrete amount is stated in
+ * the AGB (Liechtenstein/Swiss law) — the admin UI surfaces this; verify with a
+ * Treuhänder. See docs/crm-disclaimers / [[feedback_legal_financial_examples_only]].
+ */
+exports.up = async function (knex) {
+  if (!(await knex.schema.hasTable('app_settings'))) return;
+  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' },
+  ];
+  for (const s of seeds) {
+    const exists = await knex('app_settings').where({ setting_key: s.setting_key }).first();
+    if (!exists) await knex('app_settings').insert({ ...s, updated_at: new Date() });
+  }
+};
+
+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'])
+    .del();
+};