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/tests/test_widgets.py

@@ -991,3 +991,200 @@ def test_widget_reference_update_sort_order(widget_ref_client):
     # Widget instances themselves are unchanged.
     assert store.get_widget(widget_a["id"])["sort_order"] == 0
     assert store.get_widget(widget_b["id"])["sort_order"] == 1
+
+
+# ---------------------------------------------------------------------------
+# Prometheus gauge + mean adapter tests (SC-109..SC-114)
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_prometheus_gauge_adapter_returns_scalar():
+    """SC-109: gauge kind hits /api/v1/query and returns {value, thresholds}."""
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090", "timeout_seconds": 5},
+    )
+    payload = SimpleNamespace(
+        raise_for_status=lambda: None,
+        json=lambda: {
+            "data": {
+                "result": [
+                    {"metric": {"__name__": "cpu"}, "value": [100, "0.75"]},
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload) as mock_get:
+        result = await adapter.fetch(
+            service,
+            "gauge",
+            {
+                "promql": "cpu_usage",
+                "warn_at": 0.8,
+                "crit_at": 0.95,
+                "unit": "%",
+            },
+        )
+    call = mock_get.call_args
+    assert call.args[0].endswith("/api/v1/query")
+    assert call.kwargs["params"]["query"] == "cpu_usage"
+    assert result["value"] == 0.75
+    assert result["warn_at"] == 0.8
+    assert result["crit_at"] == 0.95
+    assert result["unit"] == "%"
+
+
+@pytest.mark.asyncio
+async def test_prometheus_gauge_adapter_rejects_multi_series():
+    """SC-111: gauge must be scalar-only; multi-series returns error."""
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090"},
+    )
+    payload = SimpleNamespace(
+        raise_for_status=lambda: None,
+        json=lambda: {
+            "data": {
+                "result": [
+                    {"metric": {"instance": "a"}, "value": [100, "1"]},
+                    {"metric": {"instance": "b"}, "value": [100, "2"]},
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload):
+        result = await adapter.fetch(service, "gauge", {"promql": "up"})
+    assert "error" in result
+    assert "single-series" in result["error"].lower()
+
+
+@pytest.mark.asyncio
+async def test_prometheus_gauge_adapter_requires_promql():
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090"},
+    )
+    result = await adapter.fetch(service, "gauge", {"promql": ""})
+    assert result == {"error": "promql is required"}
+
+
+@pytest.mark.asyncio
+async def test_prometheus_mean_adapter_computes_average():
+    """SC-112: mean kind averages non-null values over the window."""
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090", "timeout_seconds": 5},
+    )
+    payload = SimpleNamespace(
+        raise_for_status=lambda: None,
+        json=lambda: {
+            "data": {
+                "result": [
+                    {
+                        "metric": {"__name__": "cpu"},
+                        "values": [[100, "1.0"], [130, "2.0"], [160, "3.0"]],
+                    }
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload):
+        result = await adapter.fetch(service, "mean", {"promql": "cpu", "window": "1h"})
+    assert result["value"] == 2.0
+    assert result["unit"] is None
+
+
+@pytest.mark.asyncio
+async def test_prometheus_mean_adapter_rejects_multi_series():
+    """SC-114: mean must be scalar-only; multi-series returns error."""
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090"},
+    )
+    payload = SimpleNamespace(
+        raise_for_status=lambda: None,
+        json=lambda: {
+            "data": {
+                "result": [
+                    {"metric": {"instance": "a"}, "values": [[100, "1"]]},
+                    {"metric": {"instance": "b"}, "values": [[100, "2"]]},
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload):
+        result = await adapter.fetch(service, "mean", {"promql": "up", "window": "1h"})
+    assert "error" in result
+    assert "single-series" in result["error"].lower()
+
+
+@pytest.mark.asyncio
+async def test_prometheus_mean_adapter_skips_nan_values():
+    """SC-112: NaN / Inf values are excluded from the mean computation."""
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090"},
+    )
+    payload = SimpleNamespace(
+        raise_for_status=lambda: None,
+        json=lambda: {
+            "data": {
+                "result": [
+                    {
+                        "metric": {},
+                        "values": [[100, "2.0"], [130, "NaN"], [160, "4.0"]],
+                    }
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload):
+        result = await adapter.fetch(service, "mean", {"promql": "up", "window": "1h"})
+    # (2.0 + 4.0) / 2 = 3.0 (NaN excluded)
+    assert result["value"] == 3.0
+
+
+@pytest.mark.asyncio
+async def test_prometheus_mean_adapter_requires_promql():
+    from media_library_viewer_api.widgets.sources import PrometheusWidgetSource
+
+    adapter = PrometheusWidgetSource()
+    service = ServiceRecord(
+        id="s",
+        service_type="prometheus",
+        name="p",
+        config={"base_url": "http://p:9090"},
+    )
+    result = await adapter.fetch(service, "mean", {"promql": ""})
+    assert result == {"error": "promql is required"}