observability configs: loki delete_request_store, prometheus targets, grafana root URL

- loki: recent releases require compactor.delete_request_store when
  retention is enabled (was crash-looping on start)
- prometheus: scrape targets carried legacy dankclaw.local names with
  URL schemes (invalid); point at published LAN ports and the gitea
  quadlet's published port instead
- grafana: GF_SERVER_ROOT_URL to the canonical HTTPS name
- tests: 3 new seams for the deployed observability configs

dankclaw/compose.yaml

@@ -124,7 +124,7 @@ services:
     container_name: grafana
     restart: unless-stopped
     environment:
-      GF_SERVER_ROOT_URL: "http://grafana.dankclaw.local"
+      GF_SERVER_ROOT_URL: "https://grafana.franksdank.cloud"
       GF_SECURITY_ADMIN_USER: "${GRAFANA_ADMIN_USER}"
       GF_SECURITY_ADMIN_PASSWORD: "${GRAFANA_ADMIN_PASSWORD}"
     ports: