fix: add dedicated location /openapi.json block to return spec JSON instead of SPA HTML

The nginx catch-all location / used proxy_pass $litellm_backend_url/ which
stripped the URI path, causing /openapi.json to return the Swagger UI HTML
page instead of the actual OpenAPI spec JSON. This broke the Swagger UI
rendering with "Unable to render this definition" error.

Fix: Add a dedicated location /openapi.json block before the catch-all /
that preserves the full path, so LiteLLM returns its valid openapi: 3.1.0
spec JSON at the public endpoint.

nginx/nginx.conf

@@ -140,6 +140,22 @@ http {
             proxy_set_header Host $host;
         }
 
+        # Dedicated /openapi.json block — must come before catch-all /
+        # LiteLLM serves valid openapi: 3.1.0 JSON at this path internally,
+        # but the catch-all location / strips the URI path. This block
+        # preserves the full path so the spec JSON is returned instead of
+        # the Swagger UI SPA HTML.
+        location /openapi.json {
+            proxy_pass $litellm_backend_url/openapi.json;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header Authorization $http_authorization;
+            proxy_connect_timeout 10s;
+            proxy_read_timeout 600s;
+            proxy_buffering off;
+        }
+
         location / {
             proxy_pass $litellm_backend_url/;
             proxy_http_version 1.1;