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

client/src/contexts/AuthContext.jsx

@@ -37,6 +37,14 @@ export function AuthProvider({ children }) {
     return data.user
   }, [])
 
+  // Step 2 for SSO logins whose account has 2FA on. The pending challenge lives in
+  // an httpOnly cookie, so only the code is sent. Returns { user, returnTo }.
+  const ssoLoginTotp = useCallback(async (code) => {
+    const data = await api.ssoLoginTotp(code)
+    setUser(data.user)
+    return data
+  }, [])
+
   const logout = useCallback(async () => {
     try {
       await api.logout()
@@ -46,7 +54,7 @@ export function AuthProvider({ children }) {
   }, [])
 
   return (
-    <AuthContext.Provider value={{ user, loading, login, loginTotp, logout, refresh }}>
+    <AuthContext.Provider value={{ user, loading, login, loginTotp, ssoLoginTotp, logout, refresh }}>
       {children}
     </AuthContext.Provider>
   )