S-15c · Enforce MFA on the medewerker (Keycloak) realm (#158)

Closes #132.

Staff logins (behandel + beheer portals) now need a second factor; the citizen realms are unchanged.

**How:** every seeded medewerker carries a TOTP credential, which activates Keycloak's stock *conditional OTP* step in both the browser flow and the direct grant — no custom browser-flow JSON in the export. `CONFIGURE_TOTP` is a default required action so a medewerker added later must enrol first. ADR-0031 records the choice and, explicitly, that the shared fixture secret is a demo posture only.

**Tests (red first, 30c5279):**
- `check_realms.py` asserts the medewerker password-only grant is **refused**, then that password + TOTP succeeds and still carries the `behandelaar` role. It failed with `[MFA NOT ENFORCED]` against the old export.
- The three medewerker e2e logins move to `loginMedewerker()` (`tests/e2e/medewerker-login.ts`), which submits Keycloak's OTP prompt. Both TOTP implementations (Python `hmac`, Node `crypto`) are ~6 lines of RFC 6238 — no new dependency.

Verified locally against Keycloak 26.1: password-only → `invalid_grant`, password + code → 200, and the browser flow's `#otp` prompt accepts a computed code and issues an auth code.

## Definition of Done
- [x] Failing test/verify committed first; implementation makes it pass.
- [x] Conventional Commits referencing the issue (`refs #132`).
- [ ] CI green (verify-stack compose smoke + relevant checks).
- [x] `docker compose up` reaches green health within 3 minutes (Keycloak change is import-time only).
- [x] Docs touched (runbook, synthetic-data, demo-script) + ADR-0031 + demo note.
- [x] Closed by the merging PR (`closes #132`).

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #158

docs/synthetic-data.md

@@ -19,6 +19,11 @@ All test users share the password **`test123`**.
 | `eidas` | eIDAS (EU) | `pierre-dupont` | `eidas_id` = `FR/NL/AB-1234-5678` |
 | `medewerker` | Internal staff | `merel-behandelaar` | role `behandelaar` |
 | `medewerker` | Internal staff | `tom-teamlead` | roles `behandelaar`, `teamlead` |
+| `medewerker` | Internal staff | `bram-beheerder` | role `beheerder` |
+
+`medewerker` users additionally need a **second factor**: that realm enforces MFA (S-15c,
+ADR-0031). All three share the fixture TOTP secret `BIGMEDEWERKEROTPSEED`; print a current
+code with `python3 infra/keycloak/check_realms.py otp`.
 
 The identifying claims are injected via OIDC protocol mappers on `big-portal`
 (user-attribute → token claim); `medewerker` roles appear in `realm_access.roles`.
@@ -32,5 +37,8 @@ curl -s -X POST \
   -d username=jan-burger -d password=test123 -d scope=openid | jq -r .access_token
 ```
 
+For a `medewerker` user, add `-d totp=$(python3 infra/keycloak/check_realms.py otp)` —
+without it the grant is refused with `invalid_grant`.
+
 Decode the JWT payload to see the `bsn` claim. `make keycloak-smoke` checks every realm
 automatically.