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/integrations/__tests__/navEntries.test.ts

@@ -21,13 +21,9 @@ describe("navEntries", () => {
 
 	it("returns all observability entries", () => {
 		const entries = configuredNavEntries(
-			new Set(["alertmanager", "grafana", "prometheus"]),
+			new Set(["alertmanager", "prometheus"]),
 		);
-		expect(entries.map((e) => e.label)).toEqual([
-			"Alertmanager",
-			"Grafana",
-			"Prometheus",
-		]);
+		expect(entries.map((e) => e.label)).toEqual(["Alertmanager", "Prometheus"]);
 	});
 
 	it("returns Backups + Authentik when configured", () => {