headscale: move the tailnet to orbit.sol, route all DNS through pihole

Three connected changes, all triggered by the same outage.

base_domain leaves mgaction.town. That zone has a wildcard A+AAAA pointing
at neptun, and DNS wildcards match multi-label names, so
jupiter.hosts.mgaction.town resolved publicly to NEPTUN and Caddy proxied
to itself -- a silent loop rather than a lookup failure. Nesting the
tailnet inside the LAN domain as orbit.sol keeps the theme and resolves
unambiguously, since tailscale matches routes by longest suffix.

override_local_dns = true with pihole as the only global nameserver, so
roaming devices get ad blocking and .sol names off-LAN. With it false,
globalResolvers land in the netmap's FallbackResolvers, which a phone
with carrier DNS never consults. No public fallback is listed on purpose:
tailscale treats the list as a set, so a second entry would let queries
slip past the filter whenever mercury is slow. The cost is that mercury
is now a single point of failure for tailnet DNS.

neptun and mercury opt out individually. mercury would otherwise resolve
through itself. neptun must not depend on a Pi behind a domestic line to
renew the certificates for the control server every other node needs --
and it is circular besides, since tailscaled has to resolve
vpn.mgaction.town to connect at all. Instead neptun runs a dnsmasq stub
forwarding just orbit.sol to MagicDNS on 100.100.100.100, which tailscaled
answers whenever it is running regardless of --accept-dns. That resolves
jupiter live, so the hardcoded /etc/hosts pin is gone.

Also sets dns.nameservers.split explicitly: nixpkgs renders its own
dns.split option one level too high, but headscale reads
dns.nameservers.split (hscontrol/types/config.go:722) and so does
headplane, whose DNS page dies on the missing key with "Cannot convert
undefined or null to object". The module's option is dead as written.

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

hosts/neptun/configuration.nix

@@ -43,6 +43,29 @@
   networking.defaultGateway6 = { address = "fe80::1"; interface = "eth0"; };
   networking.nameservers = [ "9.9.9.9" "1.1.1.1" "2620:fe::fe" ];
 
+  # ---- Local split-DNS stub ----
+  # neptun must NOT take the tailnet's DNS: headscale points every node at
+  # pihole on mercury, and making a public reverse proxy's name resolution
+  # depend on a Pi behind a domestic line would take ACME renewals — and so
+  # the certs for the control server every node needs — down with it. It is
+  # also circular, since tailscaled has to resolve vpn.mgaction.town to
+  # connect in the first place.
+  #
+  # So neptun opts out with --accept-dns=false and does its own split DNS.
+  # tailscaled still answers MagicDNS on 100.100.100.100 whenever it is
+  # running (--accept-dns only governs whether it rewrites resolv.conf), so
+  # dnsmasq forwards just the tailnet suffix there and everything else to the
+  # public resolvers above. jupiter's address is therefore resolved live and
+  # never pinned — nothing to update when the tailnet is rebuilt.
+  #
+  # resolveLocalQueries (default) points resolv.conf at 127.0.0.1 and feeds
+  # networking.nameservers to dnsmasq as upstreams via resolvconf.
+  services.tailscale.extraUpFlags = [ "--accept-dns=false" ];
+  services.dnsmasq = {
+    enable = true;
+    settings.server = [ "/orbit.sol/100.100.100.100" ];
+  };
+
   # firewall (enable + 22), caddy (80/443), tailscale (trust tailscale0 + join
   # headscale) come from ../../common.nix and ../../services/{caddy,tailscale}.nix.
 
@@ -63,24 +86,24 @@
 
   # ---- Audiobookshelf ----
   services.caddy.virtualHosts."abs.mgaction.town".extraConfig = ''
-    reverse_proxy http://jupiter.hosts.mgaction.town:8000
+    reverse_proxy http://jupiter.orbit.sol:8000
   '';
 
   # ---- Seerr ----
   services.caddy.virtualHosts."seerr.mgaction.town".extraConfig = ''
-    reverse_proxy http://jupiter.hosts.mgaction.town:5055
+    reverse_proxy http://jupiter.orbit.sol:5055
   '';
 
   # ---- Jellyfin ----
   services.caddy.virtualHosts."jellyfin.mgaction.town".extraConfig = ''
-    reverse_proxy http://jupiter.hosts.mgaction.town:8096
+    reverse_proxy http://jupiter.orbit.sol:8096
   '';
 
   # ---- Gitea WebUI ----
   # Gitea's web UI and HTTPS clones (services/dev/gitea.nix, HTTP_PORT 3000).
   # Its SSH side is the separate :2222 forward further down.
   services.caddy.virtualHosts."git.mgaction.town".extraConfig = ''
-    reverse_proxy http://jupiter.hosts.mgaction.town:3000
+    reverse_proxy http://jupiter.orbit.sol:3000
   '';
 
   # ---- Gitea SSH forward ----
@@ -95,7 +118,7 @@
     wantedBy = [ "multi-user.target" ];
     serviceConfig = {
       DynamicUser = true;
-      ExecStart = "${pkgs.socat}/bin/socat TCP-LISTEN:2222,fork,reuseaddr TCP:jupiter.hosts.mgaction.town:2222";
+      ExecStart = "${pkgs.socat}/bin/socat TCP-LISTEN:2222,fork,reuseaddr TCP:jupiter.orbit.sol:2222";
       Restart = "always";
     };
   };