Adds services/forgejo-runner.nix as a host-gated module on the mediaserver
and switches the flake-update workflow from runs-on: ubuntu-latest to the
self-hosted fred-nix label, mapped to catthehacker/ubuntu:act-latest for
GitHub-action compatibility. Token lives at /var/secrets/forgejo-runner-token
so it stays out of the Nix store.
Also drops the stray result/ build symlink from the worktree.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>