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/middleware/usage.js

@@ -83,7 +83,7 @@ export function usageMiddleware(req, res, next) {
     }
     const record = usage.get(key);
     if (record && record.monthKey === monthKey && record.count >= FREE_TIER_LIMIT) {
-        res.status(429).json({ error: "Free tier limit reached (100/month). Upgrade to Pro at https://docfast.dev/#pricing for 5,000 PDFs/month." });
+        res.status(429).json({ error: "Account limit reached (100/month). Upgrade to Pro at https://docfast.dev/#pricing for 5,000 PDFs/month." });
         return;
     }
     trackUsage(key, monthKey);