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/demo-script.md

@@ -140,7 +140,8 @@ zaaktype cache). Store is in-memory: an edit reverts to the configured env on re
 
 ```bash
 make up
-# 1. Log in as bram-beheerder / test123 → "Default-fill" tab → change a value → Opslaan.
+# 1. Log in as bram-beheerder / test123 + OTP (`python3 infra/keycloak/check_realms.py otp`)
+#    → "Default-fill" tab → change a value → Opslaan.
 open http://localhost:8143/default-fill
 #
 # 2. Automated: the ACL uses the current default-fill per zaak (unit) and the endpoints are behind the
@@ -161,7 +162,8 @@ directly (ADR-0025); managing the default-fill config (S-15b) and MFA (S-15c) co
 
 ```bash
 make up
-# 1. Log in as bram-beheerder / test123 → the catalogus lists the published zaaktypen.
+# 1. Log in as bram-beheerder / test123 + OTP (`python3 infra/keycloak/check_realms.py otp`)
+#    → the catalogus lists the published zaaktypen.
 open http://localhost:8143
 #
 # 2. Automated (a CI verify-stack e2e): a beheerder logs in and sees BIG-REGISTRATIE.
@@ -304,7 +306,8 @@ make verify-local     # → "OK — a fresh local stack completed the flow with
 
 # 3. Or by hand in the browser: log in at http://localhost:8140 (jan-burger / test123), submit +
 #    upload a PDF, then approve it in the werkbak at http://localhost:8142 (merel-behandelaar /
-#    test123); it shows as INGESCHREVEN in the openbaar register at http://localhost:8141.
+#    test123 + OTP, see S-15c); it shows as INGESCHREVEN in the openbaar register at
+#    http://localhost:8141.
 ```
 
 > The zaaktype is discovered by the ACL itself since S-27 (below); `local-seed`'s `acl.env` now
@@ -589,7 +592,7 @@ or **afwijzen** — which also completes the Beoordelen task so the process adva
 
 ```text
 # 1. Open the behandel portal and log in as a behandelaar (medewerker realm):
-#    http://localhost:8142/   →  merel-behandelaar / test123
+#    http://localhost:8142/   →  merel-behandelaar / test123 + OTP
 #
 # 2. The werkbak lists the registrations awaiting beoordeling (referentie / bsn / status).
 #    Find the reference from the submit confirmation and click "Goedkeuren" on that row.
@@ -812,3 +815,32 @@ make verify-domain     # → "the timed-out registration's zaak was cancelled to
 `POST /annuleringen` → ZGW `resultaten` + `statussen` (Geannuleerd); the aggregate then moves to
 `Verlopen`. The ACL cancels the zaak **before** the aggregate is expired, so a failed ZGW call leaves the
 job for redelivery rather than diverging the two (ADR-0019).
+
+---
+
+## S-15c — MFA on the medewerker realm (#132, ADR-0031)
+
+**Outcome:** staff logins (behandel + beheer portals) need a **second factor**. The medewerker realm
+seeds every medewerker with a TOTP credential, so Keycloak's conditional-OTP step challenges them in
+both the browser flow and the direct grant; a password alone no longer yields a token. `CONFIGURE_TOTP`
+is a default required action, so a medewerker added later must enrol first. Citizen realms (digid,
+eherkenning, eidas) are unchanged — they mock brokers that carry their own assurance.
+
+```bash
+# 1. Manual: log in to the behandel portal. After username + password Keycloak asks for a code.
+python3 infra/keycloak/check_realms.py otp   # a valid code, right now
+open http://localhost:8142                   # merel-behandelaar / test123 + that code
+#
+# 2. Automated: the realm smoke check asserts the password alone is REFUSED, then that
+#    password + TOTP succeeds and still carries the behandelaar role:
+make keycloak-smoke    # → "medewerker  merel-behandelaar  password-only login refused [OK]"
+#
+# 3. End-to-end: every staff login in the e2e goes through the OTP prompt (loginMedewerker):
+make verify-e2e        # → registration.spec (behandelaar approves), catalogus.spec, default-fill.spec
+```
+
+**The path:** the seeded `otp` credential in `infra/keycloak/realms/medewerker-realm.json` activates
+Keycloak's stock conditional-OTP subflow — no custom browser flow. The fixture secret is shared and
+committed on purpose so the checks can compute codes; a real deployment enrols per-user authenticators
+(ADR-0031).
+