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.ts

@@ -3,6 +3,8 @@ import { AlertmanagerAlertsWidget } from "../widgets/AlertmanagerAlertsWidget";
 import { BackupsWidget } from "../widgets/BackupsWidget";
 import { GrafanaLinkWidget } from "../widgets/GrafanaLinkWidget";
 import { PrometheusChartWidget } from "../widgets/PrometheusChartWidget";
+import { PrometheusGaugeWidget } from "../widgets/PrometheusGaugeWidget";
+import { PrometheusMeanWidget } from "../widgets/PrometheusMeanWidget";
 import { JellyfinWidget } from "../widgets/JellyfinWidget";
 import { JellyfinNowPlayingWidget } from "../widgets/JellyfinNowPlayingWidget";
 import { PrometheusMetricWidget } from "../widgets/PrometheusMetricWidget";
@@ -130,6 +132,53 @@ export const SERVICE_REGISTRY: Record<string, ServiceBinding> = {
 				},
 				component: PrometheusChartWidget,
 			},
+			{
+				kind: "gauge",
+				name: "Gauge",
+				description:
+					"Instant query rendered as a gauge with optional threshold bands.",
+				refreshIntervalMs: 30_000,
+				defaultConfig: { promql: "" },
+				configSchema: {
+					type: "object",
+					properties: {
+						promql: {
+							type: "string",
+							description: "PromQL instant query (must return a single scalar)",
+						},
+						warn_at: { type: "number", description: "Warning threshold" },
+						crit_at: { type: "number", description: "Critical threshold" },
+						min: { type: "number" },
+						max: { type: "number" },
+						unit: { type: "string" },
+					},
+					required: ["promql"],
+				},
+				component: PrometheusGaugeWidget,
+			},
+			{
+				kind: "mean",
+				name: "Mean",
+				description: "Average value of a PromQL query over a time window.",
+				refreshIntervalMs: 60_000,
+				defaultConfig: { promql: "", window: "1h" },
+				configSchema: {
+					type: "object",
+					properties: {
+						promql: {
+							type: "string",
+							description: "PromQL range query (must return a single series)",
+						},
+						window: {
+							type: "string",
+							description: "Time window preset (1h, 6h, 24h, 7d)",
+						},
+						unit: { type: "string" },
+					},
+					required: ["promql"],
+				},
+				component: PrometheusMeanWidget,
+			},
 		],
 	},
 	jellyfin: {