Document rate limit headers in OpenAPI spec

- Add reusable header components (X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, Retry-After)
- Reference headers in 200 responses on all conversion and demo endpoints
- Add Retry-After header to 429 responses
- Update Rate Limits section in API description to mention response headers
- Add comprehensive tests for header documentation (21 new tests)
- All 809 tests passing

dist/services/email.js

@@ -14,10 +14,8 @@ const transportConfig = {
     greetingTimeout: 5000,
     socketTimeout: 10000,
     tls: { rejectUnauthorized: false },
+    ...(smtpUser && smtpPass ? { auth: { user: smtpUser, pass: smtpPass } } : {}),
 };
-if (smtpUser && smtpPass) {
-    transportConfig.auth = { user: smtpUser, pass: smtpPass };
-}
 const transporter = nodemailer.createTransport(transportConfig);
 export async function sendVerificationEmail(email, code) {
     try {