Per-host SSO: Node auth endpoints + Redis session (#57)

Adds the /__proxy_auth OIDC flow served on every proxied host:
- routes/host_auth.js: /start (PKCE+state, per-host redirect_uri), /callback
  (exchange, enforce the host allow-list via utils/host_sso.identityAllowed,
  mint session + set __proxy_sso cookie), /logout.
- models/sso_session.js: SsoSession (Redis-backed, TTL'd; read directly by the
  Lua gate) and HostSsoState (in-flight auth request).
- utils/oidc.js: per-host redirect_uri override on buildAuthUrl/exchangeCode.
- conf.hostSso (reuses conf.oidc). Allow-list logic unit-tested.

Enforcement (Lua gate + nginx location) lands next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nodejs/utils/host_sso.js

@@ -0,0 +1,35 @@
+'use strict';
+
+/**
+ * Pure authorization check for per-host SSO (#57).
+ *
+ * After the OIDC dance, the callback decides whether the authenticated identity
+ * may access the host, based on the host's allow-lists. Enforcing here (at
+ * session creation) keeps the OpenResty side simple — the Lua gate only has to
+ * confirm a valid session exists for the host.
+ *
+ * Semantics: empty allow-lists mean "any authenticated user". Otherwise the
+ * identity is allowed if its username OR email is in sso_allow_users, or any of
+ * its groups is in sso_allow_groups. All comparisons are case-insensitive.
+ */
+function identityAllowed(identity, allowUsers, allowGroups){
+	identity = identity || {};
+	allowUsers = Array.isArray(allowUsers) ? allowUsers : [];
+	allowGroups = Array.isArray(allowGroups) ? allowGroups : [];
+
+	if(allowUsers.length === 0 && allowGroups.length === 0) return true;
+
+	let lc = s => String(s).trim().toLowerCase();
+
+	let ids = [identity.username, identity.email].filter(Boolean).map(lc);
+	let users = allowUsers.map(lc);
+	if(ids.some(id => users.includes(id))) return true;
+
+	let groups = (Array.isArray(identity.groups) ? identity.groups : []).map(lc);
+	let allow = allowGroups.map(lc);
+	if(groups.some(g => allow.includes(g))) return true;
+
+	return false;
+}
+
+module.exports = {identityAllowed};