grafana: fix datasource UIDs for VictoriaMetrics migration

Update all dashboard datasource references from "prometheus" to
"victoriametrics" to match the declared datasource UID. Enable
prune and deleteDatasources to clean up the old Prometheus
(monitoring01) datasource from Grafana's database.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/grafana/default.nix

@@ -37,6 +37,10 @@
     # Declarative datasources
     provision.datasources.settings = {
       apiVersion = 1;
+      prune = true;
+      deleteDatasources = [
+        { name = "Prometheus (monitoring01)"; orgId = 1; }
+      ];
       datasources = [
         {
           name = "VictoriaMetrics";