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/widgets/sources.py

@@ -225,28 +225,13 @@ class PrometheusWidgetSource:
             timeout = int(service.config.get("timeout_seconds") or 10)
             if widget_kind == "chart":
                 return await self._fetch_chart(base_url, timeout, config)
+            if widget_kind == "gauge":
+                return await self._fetch_gauge(base_url, timeout, config)
+            if widget_kind == "mean":
+                return await self._fetch_mean(base_url, timeout, config)
             # Default: instant-query metric path (unchanged).
-            promql = config.get("promql")
-            if not promql:
-                return {"error": "promql is required"}
-            url = f"{base_url}/api/v1/query"
-            response = await asyncio.wait_for(
-                asyncio.to_thread(
-                    requests.get,
-                    url,
-                    params={"query": promql},
-                    timeout=timeout,
-                ),
-                timeout=timeout,
-            )
-            response.raise_for_status()
-            payload = response.json()
-            return {"result": payload.get("data", {})}
-        except asyncio.TimeoutError:
-            return {"error": "Widget data fetch timed out"}
-        except requests.RequestException as exc:
-            logger.exception("prometheus adapter failed")
-            return {"error": f"Prometheus query failed: {exc}"}
+            raw = await self._instant_query(base_url, timeout, config.get("promql", ""))
+            return raw
         except Exception as exc:
             logger.exception("prometheus adapter failed")
             return {"error": f"Prometheus query failed: {exc}"}
@@ -281,6 +266,34 @@ class PrometheusWidgetSource:
         result = payload.get("data", {}).get("result", [])
         return {"matrix": result}
 
+    async def _instant_query(self, base_url: str, timeout: int, promql: str) -> dict[str, Any]:
+        """Run a Prometheus ``/api/v1/query`` instant query.
+
+        Shared by the ``metric`` and ``gauge`` widget kinds. Returns
+        ``{"result": data}`` on success or ``{"error": str}`` (never raises,
+        per SC-103).
+        """
+        if not promql:
+            return {"error": "promql is required"}
+        try:
+            response = await asyncio.wait_for(
+                asyncio.to_thread(
+                    requests.get,
+                    f"{base_url}/api/v1/query",
+                    params={"query": promql},
+                    timeout=timeout,
+                ),
+                timeout=timeout,
+            )
+            response.raise_for_status()
+            payload = response.json()
+        except asyncio.TimeoutError:
+            return {"error": "Prometheus query timed out"}
+        except requests.RequestException as exc:
+            logger.exception("prometheus instant query failed")
+            return {"error": f"Prometheus query failed: {exc}"}
+        return {"result": payload.get("data", {})}
+
     async def _fetch_chart(self, base_url: str, timeout: int, config: dict[str, Any]) -> dict[str, Any]:
         """Range query → ``{series}`` for the chart widget (SC-101..SC-104)."""
         promql = config.get("promql")
@@ -292,6 +305,63 @@ class PrometheusWidgetSource:
             return raw
         return {"series": normalize_prometheus_matrix(raw["matrix"])}
 
+    async def _fetch_gauge(self, base_url: str, timeout: int, config: dict[str, Any]) -> dict[str, Any]:
+        """Instant query → scalar for the gauge widget (SC-109, SC-110, SC-111).
+
+        Scalar-only: a multi-series query returns an error (SC-111). Threshold
+        config (``warn_at``/``crit_at``/``min``/``max``/``unit``) is passed
+        through for the frontend renderer.
+        """
+        raw = await self._instant_query(base_url, timeout, config.get("promql") or "")
+        if "error" in raw:
+            return raw
+        result = raw["result"].get("result", [])
+        if len(result) != 1:
+            return {"error": "Gauge requires a single-series query; refine your PromQL"}
+        try:
+            value = float(result[0]["value"][1])
+        except (KeyError, IndexError, ValueError, TypeError):
+            return {"error": "Gauge query returned no scalar value"}
+        return {
+            "value": value,
+            "warn_at": config.get("warn_at"),
+            "crit_at": config.get("crit_at"),
+            "min": config.get("min"),
+            "max": config.get("max"),
+            "unit": config.get("unit"),
+        }
+
+    async def _fetch_mean(self, base_url: str, timeout: int, config: dict[str, Any]) -> dict[str, Any]:
+        """Range query → client-side mean for the mean widget (SC-112..SC-114).
+
+        Runs ``query_range`` over the configured window preset, averages all
+        non-null numeric samples of the single series, and returns a scalar.
+        Scalar-only: a multi-series query returns an error (SC-114).
+        """
+        promql = config.get("promql")
+        if not promql:
+            return {"error": "promql is required"}
+        window = WINDOW_PRESETS.get(config.get("window", "1h"), WINDOW_PRESETS["1h"])
+        raw = await self._range_query(base_url, timeout, promql, window)
+        if "error" in raw:
+            return raw
+        result = raw["matrix"]
+        if len(result) != 1:
+            return {"error": "Mean requires a single-series query; refine your PromQL"}
+        points = result[0].get("values") or []
+        nums: list[float] = []
+        for _, v in points:
+            if v in (None, "NaN", "+Inf", "-Inf"):
+                continue
+            try:
+                nums.append(float(v))
+            except (TypeError, ValueError):
+                continue
+        if not nums:
+            return {"error": "Mean query returned no numeric samples in the window"}
+        mean = sum(nums) / len(nums)
+        return {"value": mean, "unit": config.get("unit")}
+
 
 class AlertmanagerWidgetSource:
     """Fetch firing alerts from an Alertmanager service and summarize them."""