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/templates.js

@@ -168,6 +168,6 @@ templatesRouter.post("/:id/render", async (req, res) => {
     }
     catch (err) {
         logger.error({ err }, "Template render error");
-        res.status(500).json({ error: "Template rendering failed", detail: err.message });
+        res.status(500).json({ error: "Template rendering failed" });
     }
 });