test(infra): medewerker login must be refused without a second factor (refs #132)

The keycloak smoke check now asserts that a password-only grant on the medewerker
realm is rejected and that a TOTP code completes it. The e2e medewerker logins move
to a shared helper that submits Keycloak's OTP challenge. Both fail against the
current realm export, which enforces no MFA.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/e2e/registration.spec.ts

@@ -1,4 +1,5 @@
 import { expect, request, test } from '@playwright/test';
+import { loginMedewerker } from './medewerker-login';
 
 // Walking-skeleton happy path (S-08d + S-09 + S-09b + S-12 + S-10a + S-19b-2): a zorgprofessional
 // logs in via mock DigiD and submits through the self-service portal → BFF → domain; the entry
@@ -76,9 +77,8 @@ test('DigiD submit → public INGEDIEND → documenten → behandelaar goedkeurt
   // — the S-12 flow that replaces the temporary admin endpoint. The staff tab switches to the
   // medewerker realm (a different Keycloak realm than the citizen's digid session).
   await staff.goto('http://behandel/');
-  await staff.locator('#username').fill('merel-behandelaar');
-  await staff.locator('#password').fill('test123');
-  await staff.locator('#kc-login').click();
+  // That realm enforces MFA (S-15c), so the behandelaar logs in with password + TOTP.
+  await loginMedewerker(staff, 'merel-behandelaar');
 
   await expect(staff.getByRole('heading', { name: /Werkbak/i })).toBeVisible();