fix: grafana public dashboard URLs use localhost instead of real domain

[josecelano]

Problem

When sharing a Grafana dashboard publicly via Share → Public dashboard, Grafana
generates a URL with localhost:3000 as the base. For example, sharing the
Tracker Overview dashboard produced:

http://localhost:3000/public-dashboards/186b355b56cd482d9c441a0affdb8ecd

This URL is not reachable from outside the server.

Root Cause

Grafana builds share URLs using its root_url server setting. The default value is
http://localhost:3000/. Because GF_SERVER_ROOT_URL was not set in the Grafana
container environment, Grafana fell back to this default when constructing the
public dashboard link.

See the Grafana documentation:
Externally shared dashboards

Workaround — Manual URL Reconstruction

The access token in the generated URL is valid; only the base URL is wrong. The
corrected URL for the Tracker Overview dashboard is:

https://grafana.torrust-tracker-demo.com/public-dashboards/186b355b56cd482d9c441a0affdb8ecd

This was verified and confirmed to work. The pattern for fixing any generated URL is:

Replace	With
http://localhost:3000	https://grafana.torrust-tracker-demo.com

The remaining two dashboards still need public sharing enabled in the Grafana UI so
their access tokens can be collected:

Dashboard	File	UID	Public URL
Tracker Overview	backups/grafana/dashboards/01-tracker-overview.json	torrust-tracker-overview	(see workaround above)
UDP Tracker 1	backups/grafana/dashboards/02-udp-tracker-1.json	torrust-udp-tracker-1	(not yet enabled)
HTTP Tracker 1	backups/grafana/dashboards/03-http-tracker-1.json	torrust-http-tracker-1	(not yet enabled)

Proper Fix

Add GF_SERVER_ROOT_URL to the Grafana service in docker-compose.yml, following
the existing environment variable injection pattern — value in .env, reference in
docker-compose.yml, no hardcoded URLs.

Change 1 — server/opt/torrust/docker-compose.yml

grafana:
  environment:
    - GF_SECURITY_ADMIN_USER=${GF_SECURITY_ADMIN_USER}
    - GF_SECURITY_ADMIN_PASSWORD=${GF_SECURITY_ADMIN_PASSWORD}
    - GF_SERVER_ROOT_URL=${GF_SERVER_ROOT_URL}

Change 2 — server/opt/torrust/.env

Add the following entry in the Grafana section:

# Grafana server root URL — used to generate correct public dashboard share links
GF_SERVER_ROOT_URL='https://grafana.torrust-tracker-demo.com'

After deploying, restart the Grafana container:

docker compose up -d grafana

New public dashboard share links will automatically use the correct base URL.
Existing access tokens remain valid — only the base URL changes.

Additional Work

As part of this fix, also:

1. Enable public sharing for all three dashboards

In the Grafana UI, open each dashboard and go to Share → Public dashboard to
enable public access and obtain the access token. Collect all three final
public URLs:

Dashboard	Expected public URL pattern
Tracker Overview	https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-1>
UDP Tracker 1	https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-2>
HTTP Tracker 1	https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-3>

2. Create a combined dashboard preview image

Individual screenshots for all three dashboards already exist alongside their JSON
exports in backups/grafana/dashboards/:

Dashboard	Screenshot
Tracker Overview	backups/grafana/dashboards/01-tracker-overview.png
UDP Tracker 1	backups/grafana/dashboards/02-udp-tracker-1.png
HTTP Tracker 1	backups/grafana/dashboards/03-http-tracker-1.png

Compose those three into a single side-by-side (or grid) image and save it to
docs/media/grafana-dashboards-preview.png. This composite will be the hero image
shown in the README. The individual screenshots in backups/grafana/dashboards/
serve as detailed references and can be linked from the README section as well.

3. Update README.md

Add a Grafana Dashboards section to the README that shows the composite preview
image, links to all three public dashboards, and links to the individual detailed
screenshots in backups/grafana/dashboards/. Suggested placement: after the existing
live demo endpoints block and before the Background section.

Example structure:

## Grafana Dashboards

Live public dashboards are available without a Grafana account:

[![Grafana dashboards preview](docs/media/grafana-dashboards-preview.png)](https://grafana.torrust-tracker-demo.com)

| Dashboard        | Public link                                                                         | Screenshot                                                 |
| ---------------- | ----------------------------------------------------------------------------------- | ---------------------------------------------------------- |
| Tracker Overview | [open](https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-1>) | [view](backups/grafana/dashboards/01-tracker-overview.png) |
| UDP Tracker 1    | [open](https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-2>) | [view](backups/grafana/dashboards/02-udp-tracker-1.png)    |
| HTTP Tracker 1   | [open](https://grafana.torrust-tracker-demo.com/public-dashboards/<access-token-3>) | [view](backups/grafana/dashboards/03-http-tracker-1.png)   |