headscale: run our own DERP relay instead of Tailscale's

By default headscale fetches https://controlplane.tailscale.com/derpmap/default
at startup and treats failure as fatal, so it cannot boot when that URL is
unreachable. A self-hosted control plane that will not start without
Tailscale's infrastructure rather misses the point of self-hosting -- and
it crash-looped for exactly that reason while neptun had no DNS.

Enable the embedded DERP server on region 999 and drop the upstream map.
The relay rides Caddy on :443, which is why that vhost already sets
flush_interval -1; only STUN needs a port of its own.

Verified against headscale 0.28.0 before committing: it starts clean with
urls = [], registers "DERP region: {RegionID:999 ...}" pointing at
vpn.mgaction.town with DERPPort 443, and brings up STUN.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/vpn/headscale.nix

@@ -20,6 +20,34 @@
         base_domain = "hosts.mgaction.town";
         nameservers.global = [ "1.1.1.1" "9.9.9.9" ];
       };
+
+      # Run our own DERP relay instead of pulling Tailscale's map.
+      #
+      # With the default (urls = [controlplane.tailscale.com/derpmap/default],
+      # auto_update_enabled = true) headscale fetches that map at startup and
+      # treats failure as FATAL — so a DNS blip or a Tailscale outage stops the
+      # control server from booting at all. A self-hosted control plane that
+      # can't start without Tailscale's infrastructure rather misses the point.
+      #
+      # The relay itself rides Caddy on :443 (hence the flush_interval -1 on
+      # that vhost); only STUN needs its own UDP port.
+      derp = {
+        urls = [ ];
+        auto_update_enabled = false;
+        server = {
+          enabled = true;
+          region_id = 999;          # 900-999 is the custom range
+          region_code = "neptun";
+          region_name = "neptun";
+          stun_listen_addr = "0.0.0.0:3478";
+          automatically_add_embedded_derp_region = true;
+        };
+      };
     };
   };
+
+  # STUN for the embedded DERP server above. Also needs a matching inbound-UDP
+  # rule in netcup's edge firewall — it is stateless and defaults to denying
+  # inbound UDP outright, which silently kills every DNS/NTP reply too.
+  networking.firewall.allowedUDPPorts = [ 3478 ];
 }