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/main.py

@@ -37,6 +37,23 @@ from .version import get_backend_version, get_version_info
 logger = logging.getLogger(__name__)
 
 
+def _validate_prometheus_gateway_config() -> None:
+    """Warn (not crash) about old-shape prometheus services needing migration (GM-113)."""
+    try:
+        store = get_settings_store()
+        for service in store.list_services("prometheus"):
+            config = service.get("config") or {}
+            if "base_url" in config and "grafana_url" not in config:
+                logger.warning(
+                    "Prometheus service '%s' (id=%s) uses the old 'base_url' config shape. "
+                    "Reconfigure with grafana_url + grafana_api_key (see CHANGELOG).",
+                    service.get("name"),
+                    service.get("id"),
+                )
+    except Exception:  # pragma: no cover - startup best-effort
+        logger.exception("Failed to validate prometheus gateway config during startup")
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Application lifespan — startup/shutdown."""
@@ -58,6 +75,7 @@ async def lifespan(app: FastAPI):
         get_service_data_harness()
     except Exception:
         logger.exception("Failed to initialize service data harness during startup")
+    _validate_prometheus_gateway_config()
     mail_queue = get_mail_queue()
     backup_poller = get_backup_poller()
     mail_queue.start()