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>

hosts/http-proxy/configuration.nix

@@ -11,6 +11,22 @@
     ../../common/vm
   ];
 
+  homelab.dns.cnames = [
+    "nzbget"
+    "radarr"
+    "sonarr"
+    "ha"
+    "z2m"
+    "grafana"
+    "prometheus"
+    "alertmanager"
+    "jelly"
+    "auth"
+    "lldap"
+    "pyroscope"
+    "pushgw"
+  ];
+
   nixpkgs.config.allowUnfree = true;
   # Use the systemd-boot EFI boot loader.
   boot.loader.grub = {