feat(setup): validate setup token at step 1 before advancing

Previously "Continue" on the token step only checked the field was
non-empty; a wrong token wasn't caught until the final submit, after the
user had filled in email + password. Add a non-burning verify:

- backend: POST /setup/verify-token constant-time compares the token
  without consuming it (createInitialAdmin still claims it atomically on
  submit), gated on no-admin-exists and rate-limited like /setup/admin.
- frontend: step-1 "Continue" calls verifyToken and only advances on a
  valid token; a wrong token shows the invalidToken error on the field,
  429 -> too-many-attempts, 409 -> redirect to login.

Adds integration tests for accept-without-burn / reject / closed-once-set.

backend/src/routes/setup.js

@@ -23,6 +23,31 @@ router.get('/status', async (req, res) => {
   }
 });
 
+// Step-1 pre-flight: validate the setup token without consuming it, so the
+// two-step wizard can block "Continue" on a wrong token. Rate-limited at the
+// mount point in server.js (authRateLimiter), same as POST /admin.
+router.post('/verify-token', [
+  body('token').notEmpty().withMessage('Setup token is required'),
+], async (req, res) => {
+  const errors = validationResult(req);
+  if (!errors.isEmpty()) {
+    return res.status(400).json({ errors: errors.array() });
+  }
+  try {
+    const valid = await setupService.verifySetupToken(req.body.token);
+    if (!valid) {
+      return res.status(400).json({ error: 'Invalid setup token', field: 'token' });
+    }
+    return res.json({ valid: true });
+  } catch (err) {
+    if (err.statusCode) {
+      return res.status(err.statusCode).json({ error: err.message, field: err.details || undefined });
+    }
+    logger.error('[setup] verifyToken failed', { error: err.message });
+    return res.status(500).json({ error: 'Setup failed' });
+  }
+});
+
 router.post('/admin', [
   body('token').notEmpty().withMessage('Setup token is required'),
   body('email').isEmail().withMessage('A valid email is required'),

backend/src/services/setupService.js

@@ -80,6 +80,23 @@ function tokensMatch(provided, expected) {
   return crypto.timingSafeEqual(a, b);
 }
 
+// Pre-flight check for the two-step wizard: lets step 1 confirm the token is
+// valid before advancing to the account step, so a wrong token is caught at
+// "Continue" rather than after the user has filled in email + password. Does
+// NOT burn the token — createInitialAdmin still claims it atomically on submit.
+// Rate-limited at the mount point (same as /admin) so it can't be used to
+// brute-force the token; the token is also 24 random bytes, so guessing is
+// infeasible regardless.
+async function verifySetupToken(token) {
+  if (!(await noAdminExists())) {
+    // Setup already finished — treat the endpoint as closed (the client
+    // redirects to login on a 409).
+    throw new ConflictError('Setup already completed — an admin account exists');
+  }
+  const expected = await getAppSetting(SETUP_TOKEN_KEY);
+  return tokensMatch(token, expected);
+}
+
 // Creates the first admin as super_admin (the highest role) and returns a
 // ready-to-set admin JWT so the browser flows straight into the wizard.
 async function createInitialAdmin({ token, email, password, ip }) {
@@ -156,4 +173,4 @@ async function createInitialAdmin({ token, email, password, ip }) {
   };
 }
 
-module.exports = { getSetupStatus, ensureSetupToken, createInitialAdmin };
+module.exports = { getSetupStatus, ensureSetupToken, verifySetupToken, createInitialAdmin };