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>

system/default.nix

@@ -11,5 +11,7 @@
     ./sops.nix
     ./sshd.nix
     ./vault-secrets.nix
+
+    ../modules/homelab
   ];
 }