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.
Developer
65bae95e3c