fix(observability): 🐛 enable vmagent, fix grafana auth, disable vmauth on dev

- Enable VMAgent (was disabled → no metrics scraped)
- Remove disable_login from Grafana config; add security block so operator can auth via API
- Disable VMAuth (invalid trailing-dot hostname o12y.observability.; not needed on dev)

otc/dev.t09.de/stacks/observability/grafana-operator/manifests/grafana.yaml

@@ -35,8 +35,10 @@ spec:
     server:
       root_url: "https://grafana.dev.t09.de"
     auth:
-      disable_login: "true"
       disable_login_form: "true"
+    security:
+      admin_user: admin
+      admin_password: admin
     auth.generic_oauth:
       enabled: "true"
       name: Forgejo