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/routes/health.js

@@ -90,7 +90,7 @@ healthRouter.get("/", async (_req, res) => {
     catch (error) {
         databaseStatus = {
             status: "error",
-            message: error.message || "Database connection failed"
+            message: error instanceof Error ? error.message : "Database connection failed"
         };
         overallStatus = "degraded";
         httpStatus = 503;