## Summary
- Add Snowflake proxy as a native systemd service on ringtail (NixOS)
- Uses `pkgs.snowflake` from nixpkgs (v2.11.0)
- Hardened systemd unit with DynamicUser, ProtectSystem=strict, 512MB memory limit
- Prometheus metrics enabled on localhost:9999
## What is Snowflake?
A Tor pluggable transport that helps censored users reach the Tor network via WebRTC. **This is NOT a Tor exit node** — traffic exits through Tor exit nodes operated by others. The proxy operator cannot see traffic content (double-encrypted) and destination servers never see the proxy's IP.
## Changes
- `nixos/ringtail/configuration.nix` — new systemd service definition
- `docs/reference/services/snowflake-proxy.md` — service reference card
- `docs/reference/infrastructure/ringtail.md` — updated systemd services section
- `service-versions.yaml` — added entry (type: nixos)
## Deploy plan
After review, deploy via `mise run provision-ringtail`. Service starts automatically.
## Test plan
- [ ] `mise run provision-ringtail` succeeds
- [ ] `ssh ringtail 'systemctl status snowflake-proxy'` shows active
- [ ] `ssh ringtail 'journalctl -u snowflake-proxy --no-pager -n 20'` shows broker connections
- [ ] `ssh ringtail 'curl -s localhost:9999/metrics'` returns Prometheus metrics
Reviewed-on: #311
Erich Blume
b97e37543f