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.

backend/src/media_library_viewer_api/integrations/prometheus.py

@@ -32,6 +32,25 @@ class PrometheusChartWidgetConfig(WidgetConfigBase):
     window: str = "1h"  # one of 1h / 6h / 24h / 7d (see WINDOW_PRESETS)
 
 
+class PrometheusGaugeWidgetConfig(WidgetConfigBase):
+    """A PromQL instant query rendered as a gauge with optional threshold bands (SC-109..SC-111)."""
+
+    promql: str
+    warn_at: float | None = None
+    crit_at: float | None = None
+    min: float | None = None
+    max: float | None = None
+    unit: str | None = None
+
+
+class PrometheusMeanWidgetConfig(WidgetConfigBase):
+    """A PromQL range query averaged client-side into a single value (SC-112..SC-114)."""
+
+    promql: str
+    window: str = "1h"  # one of 1h / 6h / 24h / 7d (see WINDOW_PRESETS)
+    unit: str | None = None
+
+
 DEFINITION = ServiceDefinition(
     service_type="prometheus",
     name="Prometheus",
@@ -57,5 +76,21 @@ DEFINITION = ServiceDefinition(
             default_config={"promql": "", "window": "1h"},
             refresh_interval_ms=60_000,
         ),
+        widget_kind(
+            kind="gauge",
+            name="Gauge",
+            description="Instant query rendered as a gauge with optional threshold bands.",
+            model_cls=PrometheusGaugeWidgetConfig,
+            default_config={"promql": ""},
+            refresh_interval_ms=30_000,
+        ),
+        widget_kind(
+            kind="mean",
+            name="Mean",
+            description="Average value of a PromQL query over a time window.",
+            model_cls=PrometheusMeanWidgetConfig,
+            default_config={"promql": "", "window": "1h"},
+            refresh_interval_ms=60_000,
+        ),
     ],
 )