dns: auto-generate zone entries from host configurations

Replace static zone file with dynamically generated records:
- Add homelab.dns module with enable/cnames options
- Extract IPs from systemd.network configs (filters VPN interfaces)
- Use git commit timestamp as zone serial number
- Move external hosts to separate external-hosts.nix

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/ns/secondary-authorative.nix

@@ -1,4 +1,16 @@
-{ ... }:
+{ self, lib, ... }:
+let
+  dnsLib = import ../../lib/dns-zone.nix { inherit lib; };
+  externalHosts = import ./external-hosts.nix;
+
+  # Generate zone from flake hosts + external hosts
+  # Used as initial zone data before first AXFR completes
+  zoneData = dnsLib.generateZone {
+    inherit self externalHosts;
+    serial = self.sourceInfo.lastModified;
+    domain = "home.2rjus.net";
+  };
+in
 {
   sops.secrets.ns_xfer_key = {
     path = "/etc/nsd/xfer.key";
@@ -24,7 +36,7 @@
       "home.2rjus.net" = {
         allowNotify = [ "10.69.13.5 xferkey" ];
         requestXFR = [ "AXFR 10.69.13.5@8053 xferkey" ];
-        data = builtins.readFile ./zones-home-2rjus-net.conf;
+        data = zoneData;
       };
     };
   };