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/hooks/useObservability.ts

@@ -2,7 +2,6 @@ import { useQuery } from "@tanstack/react-query";
 import {
 	fetchAlertmanagerAlerts,
 	fetchAlertmanagerStatus,
-	fetchGrafanaStatus,
 	fetchPrometheusStatus,
 	fetchPrometheusTargets,
 	fetchMonitoringMachines,
@@ -28,16 +27,6 @@ export function useAlertmanagerStatus() {
 	});
 }
 
-export function useGrafanaStatus() {
-	return useQuery({
-		queryKey: ["observability", "grafana-status"],
-		queryFn: fetchGrafanaStatus,
-		retry: 2,
-		staleTime: 10_000,
-		refetchInterval: 30_000,
-	});
-}
-
 export function usePrometheusStatus() {
 	return useQuery({
 		queryKey: ["observability", "prometheus-status"],