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_prometheus_range.py

@@ -0,0 +1,106 @@
+"""Unit tests for the shared Prometheus range-query helpers (SC-101..SC-104)."""
+
+from __future__ import annotations
+
+import pytest
+
+from media_library_viewer_api.widgets.prometheus_range import (
+    WINDOW_PRESETS,
+    normalize_prometheus_matrix,
+    step_for_window,
+)
+
+
+class TestStepForWindow:
+    """SC-104: every preset must yield 100–300 points."""
+
+    @pytest.mark.parametrize("preset", sorted(WINDOW_PRESETS))
+    def test_presets_yield_in_band_point_counts(self, preset: str) -> None:
+        window = WINDOW_PRESETS[preset]
+        step = step_for_window(window)
+        # Clamped minimum.
+        assert step >= 15
+        point_count = window // step
+        assert 100 <= point_count <= 300, f"{preset}: {point_count} points (step={step})"
+
+    def test_floor_of_fifteen_seconds(self) -> None:
+        # A tiny window that would otherwise produce a sub-15s step is clamped.
+        assert step_for_window(60) == 15
+
+    def test_custom_target_points(self) -> None:
+        # Targeting 100 points for 1h yields step 36 (3600/100).
+        assert step_for_window(3_600, target_points=100) == 36
+
+
+class TestNormalizePrometheusMatrix:
+    """SC-102: label rule + null handling + dedup."""
+
+    def test_empty_matrix(self) -> None:
+        assert normalize_prometheus_matrix([]) == []
+
+    def test_drops_dunder_labels_and_joins(self) -> None:
+        result = [
+            {
+                "metric": {"__name__": "node_cpu_seconds_total", "instance": "host:9100", "mode": "idle"},
+                "values": [[1_700_000_000, "12.5"], [1_700_000_030, "13.0"]],
+            }
+        ]
+        out = normalize_prometheus_matrix(result)
+        assert len(out) == 1
+        assert out[0]["label"] == "instance=host:9100 mode=idle"
+        assert out[0]["points"] == [
+            {"t": 1_700_000_000, "v": 12.5},
+            {"t": 1_700_000_030, "v": 13.0},
+        ]
+
+    def test_falls_back_to_value_when_no_labels(self) -> None:
+        result = [{"metric": {}, "values": [[100, "1"]]}]
+        out = normalize_prometheus_matrix(result)
+        assert out[0]["label"] == "value"
+
+    def test_dedup_collisions_with_suffix(self) -> None:
+        # Two series with identical visible labels get a "(1)" suffix on the 2nd.
+        result = [
+            {"metric": {"job": "x"}, "values": [[1, "1"]]},
+            {"metric": {"job": "x"}, "values": [[1, "2"]]},
+        ]
+        out = normalize_prometheus_matrix(result)
+        labels = [s["label"] for s in out]
+        assert labels == ["job=x", "job=x (1)"]
+
+    def test_non_numeric_sentinels_become_none(self) -> None:
+        result = [
+            {
+                "metric": {"job": "x"},
+                "values": [
+                    [1, "NaN"],
+                    [2, "+Inf"],
+                    [3, "-Inf"],
+                    [4, "3.5"],
+                ],
+            }
+        ]
+        out = normalize_prometheus_matrix(result)
+        assert out[0]["points"] == [
+            {"t": 1, "v": None},
+            {"t": 2, "v": None},
+            {"t": 3, "v": None},
+            {"t": 4, "v": 3.5},
+        ]
+
+    def test_malformed_values_are_ignored_not_raised(self) -> None:
+        result = [
+            {
+                "metric": {"job": "x"},
+                "values": [
+                    [1, "3.5"],
+                    ["not-a-ts", "9"],  # unusable timestamp → dropped
+                    [3, "junk-value"],  # unparseable value → v: None
+                ],
+            }
+        ]
+        out = normalize_prometheus_matrix(result)
+        assert out[0]["points"] == [
+            {"t": 1, "v": 3.5},
+            {"t": 3, "v": None},
+        ]