docs(infra): document MFA on the medewerker realm + ADR-0031 (refs #132)

Runbook gains an MFA section and how to get a code; synthetic-data lists the fixture
TOTP secret and the extra grant parameter; demo-script gains the S-15c note and its
staff logins now mention the second factor. check_realms.py grows an 'otp' argument
that prints a current code for a manual demo.

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

docs/runbooks/keycloak.md

@@ -23,6 +23,9 @@ login per realm and asserts the identifying claim:
 | eidas | pierre-dupont | `eidas_id` |
 | medewerker | merel-behandelaar | role `behandelaar` |
 
+The medewerker row also asserts that the password **alone** is refused — that realm
+enforces MFA (below).
+
 All test users / credentials are in [../synthetic-data.md](../synthetic-data.md).
 
 ## Notes
@@ -35,3 +38,30 @@ All test users / credentials are in [../synthetic-data.md](../synthetic-data.md)
 - **Image** pinned to `quay.io/keycloak/keycloak:26.1`.
 - Claims are injected by OIDC protocol mappers on `big-portal` (user attribute → token
   claim); `medewerker` roles come through `realm_access.roles`.
+
+## MFA on the medewerker realm (S-15c)
+
+Staff logins (behandel + beheer portals) need a second factor; citizen/company realms
+(digid, eherkenning, eidas) do not. Two halves in `medewerker-realm.json`:
+
+- Every seeded medewerker carries a **TOTP credential** with the fixture secret
+  `BIGMEDEWERKEROTPSEED`, so Keycloak's built-in *conditional OTP* step fires on every
+  login — browser flow (an `#otp` prompt after the password) and direct grant (a `totp`
+  form field) alike.
+- `CONFIGURE_TOTP` is a **default required action**, so any medewerker added later must
+  enrol an authenticator before the first login.
+
+See [../architecture/adr-0031-mfa-on-the-medewerker-realm.md](../architecture/adr-0031-mfa-on-the-medewerker-realm.md).
+
+### Getting a code
+
+```bash
+python3 infra/keycloak/check_realms.py otp   # prints a valid 6-digit code right now
+```
+
+Or enrol a phone once: the secret in base32 is `IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE`
+(`otpauth://totp/medewerker?secret=IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE`). The e2e computes its
+own code in `tests/e2e/medewerker-login.ts`.
+
+**Fixture only.** A shared, committed secret is a demo convenience, never a production
+posture — see the ADR's consequences.