Template the NetBird hostname instead of hardcoding netbird.stoganet.com

[koinsaari]

Summary

netbird.stoganet.com is hardcoded in several places where the other hostnames use \${DOMAIN} or environment variables. This makes the repo less portable and harder to reuse.

Context

Most service hostnames in compose/Caddyfile are derived from \${DOMAIN} (e.g. jellyfin.\${DOMAIN}, seerr.\${DOMAIN}). The NetBird hostname is the exception — it appears as a literal in:

• compose/Caddyfile (site block)
• compose/netbird/config.yaml.example (exposedAddress, auth.issuer, dashboardRedirectURIs)
• compose/netbird/dashboard.env (NETBIRD_MGMT_API_ENDPOINT, AUTH_AUTHORITY)
• compose/setup.sh (--management-url)

Scope

• Introduce NETBIRD_HOSTNAME (or reuse \${DOMAIN} + a netbird subdomain) consistently across all four files
• Update the .example files to use the placeholder
• Keep the live config working through the migration

Out of scope: any change to NetBird's runtime behavior. This is purely templating.

Acceptance criteria

• No literal netbird.stoganet.com remains in tracked files (except possibly in docs/ prose)
• Fresh-host bootstrap with a different domain works end to end
• Existing deployment continues to work after pulling the change

Notes

Care needed with dashboard.env because the NetBird dashboard reads those values at container start — needs a restart of the dashboard container after the change, not just Caddy.