feat(ops): introduce (head|tail)scale server at net.tvl.fyi

This runs a headscale server on sanduny which lets users join their
machines to the TVL tailscale network.

This would theoretically let people communicate with each other on the
internal network, but also more notably joined servers can advertise
exit node capability so that we can have our own "VPN network", for
starters with endpoints in Germany, UK and Russia (whitby, sanduny and
koptevo respectively).

This setup isn't fully stable yet, notably:

* The IP range used by tailscale is just the default one right now,
  I'm not sure if that should be changed or what.

* The system is stateful (on sanduny), but the state is not (yet)
  backed up anywhere. Use with caution.

* Machine joining is a manual process requiring SSH & root access to
  sanduny.

  The process is to log in to sanduny, then get a headscale shell with
  `sudo -u headscale bash`, and to use the `headscale` CLI within
  there to administrate access.

  I've opted to create a user account `tvl` for TVL-owned machines,
  and a personal account for myself and my machines.

Change-Id: I4f1be1fe8062a6c2e77203ff72fe8709f4e4dec8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8837
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

users/tazjin/nixos/koptevo/default.nix

@@ -101,6 +101,11 @@ in
     ];
   };
 
+  services.tailscale = {
+    enable = true;
+    useRoutingFeatures = "server"; # for exit-node usage
+  };
+
   # Automatically collect garbage from the Nix store.
   services.depot.automatic-gc = {
     enable = true;