feat(prometheus-direct-charting): slice 3 — remove grafana + config rewrite + changelog

Remove the entire Grafana surface: integrations/grafana.py, GrafanaWidgetSource
(+ _fetch_chart, now redundant since prometheus chart exists), GrafanaLinkWidget,
LinksTab, get_grafana_status endpoint, useGrafanaStatus hook, GrafanaStatus type,
fetchGrafanaStatus client fn, registry/nav/tab entries (FE+BE). Rewrite
config.yaml thin-dashboard rule to match reality (recharts is sanctioned for
Prometheus-backed series). CHANGELOG migration note added.

Backend: 293 pytest pass, ruff clean. Frontend: build+lint green (0 errors).
SC-115/116 grep-clean (only prometheus_range.py migration comments + Dashboard.test
shortcut fixture remain — both spec-allowed).

frontend/src/pages/ServicesPage.tsx

@@ -548,8 +548,8 @@ export function ServicesPage() {
 				{services.length === 0 ? (
 					<Alert>
 						<AlertDescription>
-							No services yet. Add a Grafana, Prometheus, Jellyfin, Nextcloud,
-							or SSH task runner.
+							No services yet. Add a Prometheus, Jellyfin, Nextcloud, or SSH
+							task runner.
 						</AlertDescription>
 					</Alert>
 				) : (