feat(grafana-metric-gateway): slice 1 — backend gateway transport

Route all prometheus widget queries through Grafana /api/ds/query instead of
direct Prom HTTP. PrometheusConfig: drop base_url, add grafana_url +
datasource_uid; secret grafana_api_key (required). PrometheusWidgetSource →
MetricSource with _gateway_query POST method. normalize_grafana_frames
recovered from 65bae95 + shared _dedup_label helper. Gateway-path status
check. Startup old-config validation. CHANGELOG migration note. All adapter
tests rewritten for POST /api/ds/query + Grafana frames mock. Backend: 331
pytest pass, ruff clean. Frontend: build green (unchanged in S1).

backend/tests/test_prometheus_range.py

@@ -6,6 +6,8 @@ import pytest
 
 from media_library_viewer_api.widgets.prometheus_range import (
     WINDOW_PRESETS,
+    _dedup_label,
+    normalize_grafana_frames,
     normalize_prometheus_matrix,
     step_for_window,
 )
@@ -104,3 +106,152 @@ class TestNormalizePrometheusMatrix:
             {"t": 1, "v": 3.5},
             {"t": 3, "v": None},
         ]
+
+
+class TestDedupLabel:
+    """The shared label-dedup helper used by both normalizers (GM-104)."""
+
+    def test_first_use_returns_label_unchanged(self) -> None:
+        seen: dict[str, int] = {}
+        assert _dedup_label("value", seen) == "value"
+        assert seen == {"value": 0}
+
+    def test_collision_appends_suffix(self) -> None:
+        seen: dict[str, int] = {}
+        assert _dedup_label("job=x", seen) == "job=x"
+        assert _dedup_label("job=x", seen) == "job=x (1)"
+        assert _dedup_label("job=x", seen) == "job=x (2)"
+
+    def test_different_labels_dont_collide(self) -> None:
+        seen: dict[str, int] = {}
+        assert _dedup_label("a", seen) == "a"
+        assert _dedup_label("b", seen) == "b"
+
+
+class TestNormalizeGrafanaFrames:
+    """GM-104: frames normalizer recovered from 65bae95 + shared dedup."""
+
+    def test_empty_response(self) -> None:
+        assert normalize_grafana_frames({"results": {}}) == []
+        assert normalize_grafana_frames({}) == []
+
+    def test_single_frame_with_values(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {
+                            "data": {"values": [[1000, 2000], [1.5, 2.5]]},
+                            "schema": {"fields": [{"name": "Time"}, {"name": "Value"}]},
+                        }
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        assert len(out) == 1
+        assert out[0]["label"] == "Value"
+        assert out[0]["points"] == [
+            {"t": 1000, "v": 1.5},
+            {"t": 2000, "v": 2.5},
+        ]
+
+    def test_display_name_takes_priority(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {
+                            "data": {"values": [[100, 200], [0.75, 0.80]]},
+                            "schema": {
+                                "fields": [
+                                    {"name": "Time"},
+                                    {
+                                        "name": "Value",
+                                        "labels": {"instance": "host:9100"},
+                                        "config": {"displayName": "CPU Usage"},
+                                    },
+                                ]
+                            },
+                        }
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        assert out[0]["label"] == "CPU Usage"
+
+    def test_labels_fallback_when_no_display_name(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {
+                            "data": {"values": [[100], [1.0]]},
+                            "schema": {
+                                "fields": [
+                                    {"name": "Time"},
+                                    {
+                                        "name": "Value",
+                                        "labels": {"__name__": "up", "instance": "h:9100"},
+                                    },
+                                ]
+                            },
+                        }
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        assert out[0]["label"] == "instance=h:9100"
+
+    def test_falls_back_to_value_when_no_metadata(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {
+                            "data": {"values": [[100], [1.0]]},
+                            "schema": {"fields": [{"name": "Time"}, {}]},
+                        }
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        assert out[0]["label"] == "value"
+
+    def test_dedup_collisions(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {
+                            "data": {"values": [[100], [1.0]]},
+                            "schema": {"fields": [{}, {"name": "Value"}]},
+                        },
+                        {
+                            "data": {"values": [[100], [2.0]]},
+                            "schema": {"fields": [{}, {"name": "Value"}]},
+                        },
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        labels = [s["label"] for s in out]
+        assert labels == ["Value", "Value (1)"]
+
+    def test_skips_frames_with_insufficient_values(self) -> None:
+        raw = {
+            "results": {
+                "A": {
+                    "frames": [
+                        {"data": {"values": [[100]]}, "schema": {"fields": []}},
+                        {"data": {"values": [[100], [1.0]]}, "schema": {"fields": [{}, {}]}},
+                    ]
+                }
+            }
+        }
+        out = normalize_grafana_frames(raw)
+        assert len(out) == 1