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/src/media_library_viewer_api/integrations/prometheus.py

@@ -13,9 +13,10 @@ from media_library_viewer_api.integrations.base import (
 
 
 class PrometheusConfig(ServiceConfigBase):
-    """Non-secret Prometheus connection config."""
+    """Non-secret Prometheus-via-Grafana gateway config."""
 
-    base_url: ServiceBaseUrl
+    grafana_url: ServiceBaseUrl
+    datasource_uid: str = "prometheus"
     timeout_seconds: int = 10
 
 
@@ -57,7 +58,12 @@ DEFINITION = ServiceDefinition(
     description="Metrics storage and PromQL queries.",
     config_model=PrometheusConfig,
     secret_fields=[
-        SecretField(key="api_key", label="API key", helper="Optional bearer token"),
+        SecretField(
+            key="grafana_api_key",
+            label="Grafana API key",
+            required=True,
+            helper="Service account token or API key for the Grafana gateway",
+        ),
     ],
     widget_kinds=[
         widget_kind(