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/tests/test_widgets.py

@@ -667,6 +667,73 @@ async def test_grafana_adapter_chart_handles_http_failure():
     assert "failed" in result["error"].lower()
 
 
+@pytest.mark.asyncio
+async def test_prometheus_chart_adapter_runs_range_query():
+    """SC-101: chart kind hits /api/v1/query_range and returns {series}."""
+    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__": "up", "instance": "h:9100"},
+                        "values": [[100, "1"], [130, "1"]],
+                    }
+                ]
+            }
+        },
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload) as mock_get:
+        result = await adapter.fetch(service, "chart", {"promql": "up", "window": "1h"})
+
+    # query_range endpoint + window-derived start/end/step params.
+    call = mock_get.call_args
+    assert call.args[0].endswith("/api/v1/query_range")
+    params = call.kwargs["params"]
+    assert params["query"] == "up"
+    assert {"start", "end", "step"}.issubset(params)
+    # {series} shape with the shared normalization (label drops __name__).
+    assert "series" in result
+    assert result["series"][0]["label"] == "instance=h:9100"
+    assert result["series"][0]["points"] == [{"t": 100, "v": 1.0}, {"t": 130, "v": 1.0}]
+
+
+@pytest.mark.asyncio
+async def test_prometheus_chart_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, "chart", {"promql": ""})
+    assert result == {"error": "promql is required"}
+
+
+@pytest.mark.asyncio
+async def test_prometheus_chart_adapter_degrades_on_http_error():
+    """SC-103: a connection error returns {error} rather than raising."""
+    import requests as req_mod
+
+    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": 2}
+    )
+    with patch("media_library_viewer_api.widgets.sources.requests.get", side_effect=req_mod.ConnectionError("refused")):
+        result = await adapter.fetch(service, "chart", {"promql": "up", "window": "1h"})
+    assert "error" in result
+    assert "failed" in result["error"].lower()
+
+
 @pytest.mark.asyncio
 async def test_jellyfin_now_playing_filters_active_sessions():
     """now_playing should exclude idle (no NowPlayingItem) and paused sessions."""