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/__tests__/integration/workflowEngine.test.js

@@ -243,7 +243,7 @@ describe('workflow engine', () => {
     expect(wf).toBeTruthy();
     expect(!!wf.is_builtin).toBe(true);
     expect(!!wf.enabled).toBe(false);
-    expect(JSON.parse(wf.trigger_config).seedVersion).toBe(2);
+    expect(JSON.parse(wf.trigger_config).seedVersion).toBe(3);
 
     const nodes = await db('workflow_nodes').where({ workflow_id: wf.id, version: wf.version });
     expect(nodes.filter((n) => n.type === 'trigger')).toHaveLength(1);
@@ -267,7 +267,7 @@ describe('workflow engine', () => {
     await seedBuiltinWorkflowsAtBoot(db, noopLogger);
     const reseeded = await db('workflows').where({ id: wf.id }).first();
     expect(reseeded.version).toBe(wf.version + 1); // bumped
-    expect(JSON.parse(reseeded.trigger_config).seedVersion).toBe(2);
+    expect(JSON.parse(reseeded.trigger_config).seedVersion).toBe(3);
     const newNodes = await db('workflow_nodes').where({ workflow_id: wf.id, version: reseeded.version });
     expect(newNodes.some((n) => n.type === 'gate')).toBe(false); // legacy graph replaced