feat: [#415] add GF_SERVER_ROOT_URL to Grafana service in generated docker-compose

[josecelano]

Summary

Closes #415

When the deployer generates a docker-compose.yml, the Grafana service now includes GF_SERVER_ROOT_URL so that public dashboard share links use the correct external URL instead of http://localhost:3000.

Changes

The implementation follows the existing env-var injection pattern (value in .env, reference in docker-compose.yml):

• templates/docker-compose/.env.tera — emits GF_SERVER_ROOT_URL='<url>' conditionally when a Grafana domain is configured
• templates/docker-compose/docker-compose.yml.tera — adds - GF_SERVER_ROOT_URL=${GF_SERVER_ROOT_URL} to the Grafana environment: block, conditionally
• GrafanaServiceContext (infrastructure layer) — computes server_root_url from domain + use_tls_proxy flag: https://<domain> when TLS is enabled, http://<domain> otherwise, None when no domain is set
• GrafanaServiceConfig / EnvContext (infrastructure layer) — carries server_root_url through to the .env template
• apply_grafana_env_context (application layer) — derives and passes server_root_url when building the env context

Behaviour

Config	GF_SERVER_ROOT_URL emitted
No domain set	❌ omitted
domain set, use_tls_proxy=false	http://<domain>
domain set, use_tls_proxy=true	https://<domain>

Testing

All existing tests pass (cargo test — 416 tests). Pre-commit checks pass (TORRUST_TD_SKIP_SLOW_TESTS=true ./scripts/pre-commit.sh).

[josecelano]

ACK 8a269d1