grafana: fix llama-cpp annotation query format for Grafana 12

Grafana 12 expects Prometheus annotation queries wrapped in a 'target'
object with datasource, expr, refId, and range fields. The previous
format had expr/step as top-level fields which Grafana silently ignored.

services/grafana/dashboard.nix

@@ -53,8 +53,13 @@ let
         datasource = promDs;
         enable = true;
         iconColor = "purple";
-        expr = "llamacpp:requests_processing > 0";
-        step = "10s";
+        target = {
+          datasource = promDs;
+          expr = "llamacpp:requests_processing > 0";
+          instant = false;
+          range = true;
+          refId = "A";
+        };
         titleFormat = "LLM inference";
       }
     ];