feat(k8s): optionally auto-fill the medewerker OTP step for the public demo (refs #177) (#181)

## What & why

For the public demo on `big-behandel` / `big-beheer`, visitors should see MFA being enforced without needing an authenticator app. This adds an opt-in Keycloak theme that fills in and submits the medewerker OTP code itself.

- **Theme as real files in `infra/keycloak/themes/big-demo/`**, next to the realms:
  - `login/theme.properties`: `keycloak.v2` plus `scripts=js/otp-autofill.js`. I checked the 26.1 source: `keycloak.v2` loads theme `scripts` and sets none of its own.
  - `login/resources/js/otp-autofill.js`: on the OTP page, computes the code (RFC 6238, Keycloak's default policy) from the fixture secret `BIGMEDEWERKEROTPSEED` and submits it.
  - `account`, `admin`, `email`: plain children of Keycloak 26's defaults. Without them the account console returns 500 (see notes).
- **Seeded like every other file input:** `infra/helm/seed-configmaps.sh` creates the `rr-kc-theme` ConfigMap, and the chart mounts it as a directory. The podspec gains `items` so flat ConfigMap keys map to theme paths. Keycloak runs `start-dev` (no theme cache), so edits show up about a minute after a reseed.
- **Switch:** `demo.otpAutofill` only decides whether `KC_SPI_THEME_DEFAULT=big-demo` is set. `big.env` now skips env values that render empty, and no existing env var is empty. **Off, the render is identical to main except for that one missing variable,** so Keycloak keeps its stock theme. The realm JSONs are untouched, so compose and the e2e tests still require a code.
- **Single-use codes:** a second login in the same 30 s window spends the next counter, as `nextUnusedCounter` does in the e2e. Past that it only fills in the field and doesn't submit, so a rejected code can't loop.
- **Deploy workflow:** repo variable `OTP_AUTOFILL=true` → `--set demo.otpAutofill=true`. Flipping it changes the pod's env, so Keycloak restarts.

Refs #177

## Definition of Done

- [x] Linked Gitea issue (above).
- [ ] Failing test committed before the implementation. *(Not done; checks below.)*
- [x] Conventional Commits referencing the issue (`refs #NN`).
- [ ] CI green
- [x] `docker compose up` unaffected (chart only).
- [x] Docs updated (Talos runbook, "Publishing through the labs Caddy").
- [ ] ADR. The fixture-secret trade-off is ADR-0031's; this only automates typing it in.

## Notes for reviewers

- **Tested on the live cluster.** I patched the running Keycloak with the rendered theme (autofill on) and ran real headless Chromium logins against the public hosts:
  - `merel-behandelaar` on big-behandel: only username and password typed. The OTP page loaded the script, submitted by itself, and the user landed in the Werkbak.
  - `jan-burger` on big-mijn still logs in (regression check).
  - `/realms/medewerker/account/` returns 200.
- **Account console 500, found live and fixed in the second commit.** `KC_SPI_THEME_DEFAULT` applies to every theme type, and Keycloak does *not* fall back for a type the theme lacks (`NullPointerException ... "theme" is null`). `big-demo` now declares login, account, admin and email, each a plain child of Keycloak 26's default. It's one ConfigMap mounted as a directory; the podspec gains `items` for that.
- **Keycloak restarts cause about 5 minutes of BFF 401s.** This is not caused by this PR, but you'll see it whenever Keycloak restarts. Dev-mode Keycloak makes new signing keys on each boot, and the BFF refreshes its cached keys at most every 5 minutes. Seen live: 401 right after the restart, 204 about 4½ minutes later. Flipping `OTP_AUTOFILL` restarts Keycloak, so expect this briefly.
- `make k8s-lint` and `make k8s-drift` pass. The rendered script's code matches `infra/keycloak/check_realms.py otp`.
- **Security:** with it on, the public behandel and beheer portals are protected only by the committed password `test123`. That's intentional for synthetic demo data. Never enable it anywhere real.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #181

infra/keycloak/themes/big-demo/account/theme.properties

@@ -0,0 +1,4 @@
+# The chart makes big-demo the default for every theme type, and Keycloak does not
+# fall back for a type a theme lacks (the account page then fails), so each type is
+# declared as a plain child of Keycloak 26's own default.
+parent=keycloak.v3

infra/keycloak/themes/big-demo/admin/theme.properties

@@ -0,0 +1,4 @@
+# The chart makes big-demo the default for every theme type, and Keycloak does not
+# fall back for a type a theme lacks (the admin page then fails), so each type is
+# declared as a plain child of Keycloak 26's own default.
+parent=keycloak.v2

infra/keycloak/themes/big-demo/email/theme.properties

@@ -0,0 +1,4 @@
+# The chart makes big-demo the default for every theme type, and Keycloak does not
+# fall back for a type a theme lacks (the email page then fails), so each type is
+# declared as a plain child of Keycloak 26's own default.
+parent=keycloak

infra/keycloak/themes/big-demo/login/resources/js/otp-autofill.js

@@ -0,0 +1,24 @@
+// RFC 6238 with Keycloak's default policy (HmacSHA1, 6 digits, 30 s) over the
+// raw bytes of the medewerker fixture secret — same as tests/e2e/keycloak-login.ts.
+document.addEventListener('DOMContentLoaded', async () => {
+  const input = document.querySelector('input[name="otp"]');
+  if (!input || !input.form) return;
+  const key = await crypto.subtle.importKey('raw',
+    new TextEncoder().encode('BIGMEDEWERKEROTPSEED'), { name: 'HMAC', hash: 'SHA-1' }, false, ['sign']);
+  // A code is single-use, so a second login in the same window spends the next
+  // counter (Keycloak's look-ahead accepts it). Past that, fill but don't submit,
+  // so a rejected code can't turn into a submit loop.
+  const now = Math.floor(Date.now() / 30000);
+  let last = -1;
+  try { last = Number(sessionStorage.getItem('big-otp-counter')) || -1; } catch {}
+  const counter = Math.max(now, last + 1);
+  const msg = new DataView(new ArrayBuffer(8));
+  msg.setBigUint64(0, BigInt(counter));
+  const mac = new Uint8Array(await crypto.subtle.sign('HMAC', key, msg.buffer));
+  const o = mac[19] & 0x0f;
+  const n = ((mac[o] & 0x7f) << 24 | mac[o + 1] << 16 | mac[o + 2] << 8 | mac[o + 3]) % 1e6;
+  input.value = String(n).padStart(6, '0');
+  if (counter > now + 1) return;
+  try { sessionStorage.setItem('big-otp-counter', String(counter)); } catch {}
+  input.form.requestSubmit();
+});

infra/keycloak/themes/big-demo/login/theme.properties

@@ -0,0 +1,11 @@
+# Demo login theme for the public Talos deployment: keycloak.v2 plus a script that
+# fills in and submits the medewerker OTP step from the committed fixture secret
+# (docs/runbooks/keycloak.md). Only used when the chart's demo.otpAutofill is on —
+# it then becomes Keycloak's default theme. Never enable it anywhere real.
+#
+# Add styles, messages or template overrides here as in any Keycloak theme
+# (https://www.keycloak.org/ui-customization/themes); new files must also be
+# listed in infra/helm/seed-configmaps.sh and the keycloak `files` in values.yaml.
+parent=keycloak.v2
+import=common/keycloak
+scripts=js/otp-autofill.js