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/integrations/base.py

@@ -1,7 +1,7 @@
 """Base classes for service integrations.
 
 A *service definition* is a closed, compile-time description of an external service
-the app can talk to (Grafana, Jellyfin, …). Each definition declares:
+the app can talk to (Jellyfin, Prometheus, …). Each definition declares:
 
 * its non-secret ``config_schema`` (derived from a Pydantic model),
 * the secret fields it accepts (API keys / tokens),
@@ -24,7 +24,7 @@ from pydantic import BaseModel, BeforeValidator, Field
 def _validate_service_base_url(value: Any) -> str:
     """Require an absolute http(s) URL for service ``base_url`` fields.
 
-    Relative hosts (e.g. ``grafana.example.com``) break downstream HTTP clients
+    Relative hosts (e.g. ``example.com``) break downstream HTTP clients
     because ``requests`` treats them as relative paths, so we fail fast with a
     clear error instead of letting the call silently malfunction.
     """