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/widgets/index.ts

@@ -2,6 +2,8 @@ export { AlertmanagerAlertsWidget } from "./AlertmanagerAlertsWidget";
 export { BackupsWidget } from "./BackupsWidget";
 export { GrafanaLinkWidget } from "./GrafanaLinkWidget";
 export { PrometheusChartWidget } from "./PrometheusChartWidget";
+export { PrometheusGaugeWidget } from "./PrometheusGaugeWidget";
+export { PrometheusMeanWidget } from "./PrometheusMeanWidget";
 export { JellyfinWidget } from "./JellyfinWidget";
 export { PrometheusMetricWidget } from "./PrometheusMetricWidget";
 export { SshTaskWidget } from "./SshTaskWidget";