Enforce TOTP second factor on SSO login (#31)

SSO login minted a full session immediately, ignoring the account's
totp_enabled flag — so a 2FA admin with a linked Google/Discord/OIDC
identity could sign in without their authenticator code, silently
downgrading the account to single-factor (the strength of the IdP login).
The local password flow already gates on needsTotp(); SSO did not.

Wire SSO through the same staged-TOTP gate:

- ssoState: createTotpPending/verifyTotpPending + a short-lived httpOnly
  sso_totp cookie. The pending token carries stage:'totp' (session
  validation rejects it) + kind:'sso_totp' (scoped to the SSO endpoint)
  plus the resolved context (userId, provider, authMethod, returnTo).
- sso.controller: finishLogin now stages the challenge and redirects to
  /admin/login?sso_totp=1 instead of creating a session when the account
  has TOTP on. New finishSsoTotp verifies the code (backoff + bot-scoring
  on failure, mirroring loginTotp) and only then mints the session.
- sso.routes: POST /auth/sso/totp behind the same backoff/slow/limiter
  stack and code validation as the local TOTP endpoint.
- client: AdminLogin detects ?sso_totp=1 and completes over fetch via
  api.ssoLoginTotp; the challenge never touches the URL or JS.

Keeps the second factor httpOnly throughout, consistent with the SSO tx
cookie. 12 new tests; full suite 106/106.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019rao86n5cXpwAyjdBFEshV

server/test/ssoState.test.js

@@ -1,10 +1,18 @@
+// Point the DB at a closed port before requiring the auth layer: session.service
+// (used by one test below) pulls in the users model → db pool at load, and an idle
+// pool would keep this process alive. None of these tests touch the database.
 process.env.JWT_SECRET = process.env.JWT_SECRET || 'test-secret'
+process.env.DB_HOST = '127.0.0.1'
+process.env.DB_PORT = '59999'
 
-const { test } = require('node:test')
+const { test, after } = require('node:test')
 const assert = require('node:assert/strict')
 const crypto = require('crypto')
 
 const ssoState = require('../src/auth/ssoState')
+const db = require('../src/utils/db')
+
+after(() => db.close())
 
 test('createTx → verifyTx round-trips the flow payload', () => {
   const tx = ssoState.createTx({ provider: 'google', mode: 'login', returnTo: '/admin/posts' })
@@ -36,3 +44,27 @@ test('verifyTx rejects a non-tx token', () => {
   const notTx = token.signToken({ id: 1, username: 'a', role: 'admin' })
   assert.equal(ssoState.verifyTx(notTx, 'anything'), null)
 })
+
+test('createTotpPending → verifyTotpPending round-trips the SSO 2FA context', () => {
+  const pending = ssoState.createTotpPending({ userId: 7, provider: 'google', authMethod: 'google', returnTo: '/admin/posts' })
+  const payload = ssoState.verifyTotpPending(pending)
+  assert.ok(payload)
+  assert.equal(payload.id, 7)
+  assert.equal(payload.provider, 'google')
+  assert.equal(payload.authMethod, 'google')
+  assert.equal(payload.returnTo, '/admin/posts')
+  assert.equal(payload.stage, 'totp')
+})
+
+test('a pending-TOTP token is NOT accepted as a session (stage + kind reject it)', () => {
+  const sessionService = require('../src/auth/session.service')
+  const pending = ssoState.createTotpPending({ userId: 7, provider: 'google', authMethod: 'google' })
+  assert.equal(sessionService.decodeIdentity(pending), null)
+})
+
+test('verifyTotpPending rejects a plain session and a bare TOTP challenge', () => {
+  const token = require('../src/auth/token')
+  assert.equal(ssoState.verifyTotpPending(token.signToken({ id: 1, username: 'a', role: 'admin' })), null)
+  assert.equal(ssoState.verifyTotpPending(token.signTotpChallenge({ id: 1 })), null)
+  assert.equal(ssoState.verifyTotpPending(null), null)
+})