enable proxy buffering for calls to OIDC endpoints

hosting/proxy/nginx.prod.conf

@@ -106,6 +106,12 @@ http {
 
     location ~ ^/api/(system|admin|global)/ {
       proxy_set_header    Host                $host;
+
+      # Enable buffering for potentially large OIDC configs
+      proxy_buffering on;
+      proxy_buffer_size 16k;
+      proxy_buffers 4 32k;
+
       proxy_pass      $worker;
     }