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/test/unit/host_sso.test.js

@@ -0,0 +1,42 @@
+'use strict';
+
+const {describe, test} = require('node:test');
+const assert = require('node:assert');
+
+const {identityAllowed} = require('../../utils/host_sso');
+
+describe('identityAllowed (per-host SSO authorization)', () => {
+	const id = {username: 'alice', email: 'alice@x.com', groups: ['dns-team', 'staff']};
+
+	test('empty allow-lists allow any authenticated user', () => {
+		assert.strictEqual(identityAllowed(id, [], []), true);
+		assert.strictEqual(identityAllowed(id, undefined, undefined), true);
+	});
+
+	test('allows by username', () => {
+		assert.strictEqual(identityAllowed(id, ['bob', 'alice'], []), true);
+	});
+
+	test('allows by email', () => {
+		assert.strictEqual(identityAllowed(id, ['alice@x.com'], []), true);
+	});
+
+	test('allows by group membership', () => {
+		assert.strictEqual(identityAllowed(id, [], ['dns-team']), true);
+	});
+
+	test('denies when neither user nor group matches a non-empty list', () => {
+		assert.strictEqual(identityAllowed(id, ['bob'], ['admins']), false);
+	});
+
+	test('is case-insensitive', () => {
+		assert.strictEqual(identityAllowed(id, ['ALICE'], []), true);
+		assert.strictEqual(identityAllowed(id, [], ['DNS-Team']), true);
+		assert.strictEqual(identityAllowed({username: 'A', email: 'A@X.com'}, ['a@x.com'], []), true);
+	});
+
+	test('handles a missing/empty identity gracefully', () => {
+		assert.strictEqual(identityAllowed({}, ['bob'], ['admins']), false);
+		assert.strictEqual(identityAllowed({}, [], []), true);
+	});
+});