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/server.js

@@ -399,6 +399,7 @@ async function initializeRateLimiters() {
   app.use('/api/gallery/:slug/verify', authRateLimiter);
   app.use('/api/admin/auth/login', authRateLimiter);
   app.use('/api/setup/admin', authRateLimiter);
+  app.use('/api/setup/verify-token', authRateLimiter);
 }
 
 // Note: Rate limiters will be initialized after database connection