grafana.ini.j2 sets root_url to a relative path, breaking Discord (and other) webhook notifications

The grafana.ini.j2 template sets root_url to a relative path (/ui/). This causes Grafana to generate relative URLs in alert notification payloads; for example, embed URLs like "/ui/alerting/list" instead of fully qualified URLs.

Discord's webhook API rejects payloads containing invalid URLs in embed fields, returning:

webhook failed validation: the Discord API responded (status 400) with error code 0

Expected Behavior

root_url should be a full URL (e.g. https://g.pigsty/ui/) so that Grafana generates valid absolute URLs in all outgoing notifications.

Workaround

Manually edit /etc/grafana/grafana.ini after deployment:

[server]
root_url = https://your-grafana-domain/ui/

Then restart Grafana. Note this gets overwritten on the next ./infra.yml -t grafana run.

As a permanent fix, I guess we could allow one to configure this through the pigsty.yml file

1. Expose a pigsty.yml variable (e.g. grafana_root_url) that gets templated into grafana.ini.j2, defaulting to http://{{ admin_ip }}/ui/ or deriving from the infra_portal.grafana.domain definition.
2. Update the template to build root_url from existing variables like infra_portal.grafana.domain and infra_portal.grafana.endpoint.