feat(prometheus-direct-charting): slice 3 — remove grafana + config rewrite + changelog

Remove the entire Grafana surface: integrations/grafana.py, GrafanaWidgetSource
(+ _fetch_chart, now redundant since prometheus chart exists), GrafanaLinkWidget,
LinksTab, get_grafana_status endpoint, useGrafanaStatus hook, GrafanaStatus type,
fetchGrafanaStatus client fn, registry/nav/tab entries (FE+BE). Rewrite
config.yaml thin-dashboard rule to match reality (recharts is sanctioned for
Prometheus-backed series). CHANGELOG migration note added.

Backend: 293 pytest pass, ruff clean. Frontend: build+lint green (0 errors).
SC-115/116 grep-clean (only prometheus_range.py migration comments + Dashboard.test
shortcut fixture remain — both spec-allowed).

backend/src/media_library_viewer_api/routers/monitoring.py

@@ -1,6 +1,6 @@
 """Monitoring router — observability service status.
 
-Observability components (Alertmanager, Grafana, Prometheus) are resolved from
+Observability components (Alertmanager, Prometheus) are resolved from
 the service registry, not environment variables. The endpoints pick the first
 enabled instance of a type when no ``service_id`` is given, and return graceful
 "not configured" / "unreachable" payloads so the UI always renders a health card.
@@ -172,29 +172,6 @@ def get_alertmanager_status(
     }
 
 
-@router.get("/grafana-status")
-def get_grafana_status(
-    service_id: str | None = None,
-    store: SettingsStore = Depends(get_settings_store),
-) -> dict[str, Any]:
-    """Probe a Grafana service instance's ``/api/health`` endpoint."""
-    service = resolve_service_record(store, "grafana", service_id)
-    if service is None:
-        return _status_response(None, error="no_service_configured")
-    try:
-        response = requests.get(
-            f"{_base_url(service)}/api/health",
-            headers=_auth_headers(service),
-            timeout=_timeout(service, 5),
-        )
-        response.raise_for_status()
-        data = response.json()
-    except Exception:
-        logger.exception("Failed to fetch Grafana status")
-        return _status_response(service, error="grafana_unreachable")
-    return _status_response(service, version=data.get("version", ""))
-
-
 @router.get("/prometheus-status")
 def get_prometheus_status(
     service_id: str | None = None,