feat(prometheus-direct-charting): slice 1 — prom range query + chart rebrand

Add PrometheusWidgetSource._fetch_chart hitting /api/v1/query_range directly
(SC-101..104). New shared helpers in widgets/prometheus_range.py:
step_for_window (window preset -> step, ~200pts) and normalize_prometheus_matrix
(extracted label/dedup rule, retargeted at Prom matrix, robust to malformed
data). Chart widget kind moved grafana->prometheus in both registries;
GrafanaChartWidget renamed -> PrometheusChartWidget (git mv, recharts body
preserved). Grafana binding/service untouched (removed in slice 3).

Backend: 298 pytest pass, ruff clean. Frontend: 128 vitest pass, build+lint green.

backend/src/media_library_viewer_api/integrations/prometheus.py

@@ -25,6 +25,13 @@ class PrometheusMetricWidgetConfig(WidgetConfigBase):
     promql: str
 
 
+class PrometheusChartWidgetConfig(WidgetConfigBase):
+    """A PromQL range query rendered as a multi-series line chart (SC-101..SC-104)."""
+
+    promql: str
+    window: str = "1h"  # one of 1h / 6h / 24h / 7d (see WINDOW_PRESETS)
+
+
 DEFINITION = ServiceDefinition(
     service_type="prometheus",
     name="Prometheus",
@@ -42,5 +49,13 @@ DEFINITION = ServiceDefinition(
             default_config={"promql": ""},
             refresh_interval_ms=30_000,
         ),
+        widget_kind(
+            kind="chart",
+            name="Chart",
+            description="Multi-series line chart from a PromQL range query.",
+            model_cls=PrometheusChartWidgetConfig,
+            default_config={"promql": "", "window": "1h"},
+            refresh_interval_ms=60_000,
+        ),
     ],
 )