feat(keycloak): declare passkey (WebAuthn passwordless) login for the homelab realm

Adds the realm's WebAuthn Passwordless policy and restructures the browser-mfa
flow so a passkey is an ALTERNATIVE beside the password form. Additive and
optional by design: this IdP gates every SSO app plus kubectl, so a mandatory
passkey flow that misfires is a total lock-out. Enrolment is not forced.

Policy: RP ID is ${BASE_DOMAIN} (not the keycloak host) because a passkey's RP ID
is fixed at enrolment and must not be bound to a name that can move; resident key
required (a non-discoverable credential is a second factor, not a passkey); user
verification required (without it, possession alone is a complete login).

Flow: auth-username-password-form is split into auth-username-form +
auth-password-form, because the server must know who is logging in before it can
decide whether a passkey exists. Conditional OTP for platform-admins is relocated
intact onto the password branch only — a UV-required passkey is already two
phishing-resistant factors.

No requiredActions block is declared, deliberately: Keycloak 26.0.6 treats
requiredActions as REPLACE not merge, so declaring one would delete the default
set including CONFIGURE_TOTP and lock every platform-admin out of the OTP step
this flow still enforces. webauthn-register-passwordless is already registered
enabled-but-not-default by Keycloak's own defaults.

UNAPPLIED: the realm import is a one-shot resting at Done=True/Started=False, so
this reaches the live realm never. See the handoff entry and the README section.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/keycloak/README.md

@@ -154,6 +154,74 @@ NEW client, a new protocol mapper or a changed scope still needs the realm impor
 (it will not on its own) or a deliberate admin-API/`kcadm` change. `Done=True` is never evidence that
 a realm edit landed.
 
+## Passkeys (WebAuthn Passwordless) — DECLARED IN GIT, NOT YET IN THE REALM (2026-10-17)
+
+`keycloak-realm-import.yaml` now declares passkey login: the realm's **WebAuthn Passwordless**
+policy (`webAuthnPolicyPasswordless*`), and a `browser-mfa` flow that offers a passkey as an
+**ALTERNATIVE** beside the password form. Design decisions and their reasoning are in the manifest
+comments, not repeated here. The headline choices:
+
+| Decision | Value | One-line reason |
+|---|---|---|
+| Additive or replacing? | **Additive** — passkey *or* password | A misfiring mandatory flow locks the single operator out of every SSO app *and* kubectl |
+| Enrolment | **Optional**, never a default action | Same reason; `webauthn-register-passwordless` is already enabled-not-forced by default |
+| Relying-party ID | `${BASE_DOMAIN}` (not `keycloak.${BASE_DOMAIN}`) | An unre-keyable credential must not be bound to a hostname that can move |
+| User verification | `required` | Without it, mere possession of an unlocked device is a complete login |
+| Resident key | `Yes` | A non-discoverable credential is a second factor, not a passkey |
+| OTP after a passkey? | **No** — conditional OTP stays on the password branch | A UV-required passkey is already two factors, and a phishing-resistant pair at that |
+
+### This change does NOT reach the live realm. At all.
+
+Same one-shot failure this file already documents twice above, third field over. Measured on prod
+2026-10-17:
+
+```
+$ kubectl --context k3s-prod -n keycloak get keycloakrealmimport homelab -o jsonpath='{.status}'
+{"conditions":[{"status":"True","type":"Done"},{"status":"False","type":"Started"},
+               {"status":"False","type":"HasErrors"}]}
+```
+
+`Done=True` / `Started=False` — the import has completed and **will not re-run**, so a policy block
+and three new sub-flows added to this file change nothing about how anyone logs in. The
+client-secret/client-URL reconciler does **not** close this gap and was never meant to: its hard
+endpoint allow-list makes `/authentication` unreachable **by design** (see the table above), and
+realm-level policy fields are not client fields. This is exactly the "a NEW client, a new protocol
+mapper or a changed scope still needs the realm import to actually re-run" case named above.
+
+**So the manifest is the declared source of truth and a DR-rebuild input, and nothing more, until
+an operator applies it by hand.** Two paths, both operator actions:
+
+1. **Admin console (expected path).** *Authentication → Policies → WebAuthn Passwordless Policy*
+   for the policy block, then *Authentication → Flows → browser-mfa* to rebuild the executions to
+   match the manifest. The flow edit is the fiddly half: split the existing Username Password Form
+   into Username Form + Password Form and add the two sub-flows exactly as declared.
+2. **A deliberate re-import.** Deleting and recreating the `KeycloakRealmImport` re-runs it against
+   the **existing** realm. Do not reach for this casually — it is the operation whose failure mode
+   is the 2026-09-06 total SSO outage documented above, and it has not been rehearsed here.
+
+Either way the realm is then configured by hand and git is only the record of intent, which is a
+live exception to R-IDP-03 (config as code) — not a new one, but one more instance of it.
+
+### Verifying it actually works (nobody has, as of 2026-10-17)
+
+No passkey has been enrolled and no browser has been pointed at this. The checks that would prove
+it, in order:
+
+```sh
+# 1. the policy is live (RP ID must read as the base domain, NOT the keycloak host, NOT a URL)
+#    Authentication -> Policies -> WebAuthn Passwordless Policy, in the admin console.
+# 2. enrol: account console (https://keycloak.${BASE_DOMAIN}/realms/homelab/account) -> Signing in
+#    -> Passwordless -> set up. A browser/OS passkey prompt must appear; if it does not, the RP ID
+#    is wrong for the origin and NOTHING will say so.
+# 3. log in: a fresh private window to any SSO-gated app. Expect username screen, then a passkey
+#    prompt. A platform-admin must NOT be asked for TOTP on this path.
+# 4. regression (the important one): a user with NO passkey must still get the password screen,
+#    and a platform-admin must still be asked for TOTP there.
+```
+
+A failed passkey ceremony is usually silent in the server log — the browser refuses before Keycloak
+is involved — so step 2 failing with no log entry is the expected signature of a wrong RP ID.
+
 ## Exceptions that keep native auth (do not force SSO) — ADR-0014
 
 Plex/Seerr (Plex auth), Minecraft (Mojang/Microsoft), UniFi (native).