feat(prometheus-direct-charting): slice 2 — gauge + mean widgets

Add gauge widget (recharts RadialBarChart with configurable threshold
bands, scalar-only per SC-111) and mean widget (client-side average over
range-query window, scalar-only per SC-114). Extract shared _instant_query
helper from the metric path; _fetch_gauge and _fetch_mean dispatch in
PrometheusWidgetSource.fetch(). Both new widget kinds declared in
integrations/prometheus.py and frontend registry.

Backend: 305 pytest pass, ruff clean. Frontend: 136 vitest pass, build+lint green.

frontend/src/integrations/registry.test.ts

@@ -27,6 +27,8 @@ describe("service registry", () => {
 		expect(SERVICE_REGISTRY.prometheus.widgets.map((w) => w.kind)).toEqual([
 			"metric",
 			"chart",
+			"gauge",
+			"mean",
 		]);
 		expect(SERVICE_REGISTRY.alertmanager.widgets.map((w) => w.kind)).toEqual([
 			"active_alerts",