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>

modules/homelab/dns.nix

@@ -0,0 +1,20 @@
+{ config, lib, ... }:
+let
+  cfg = config.homelab.dns;
+in
+{
+  options.homelab.dns = {
+    enable = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = "Include this host in DNS zone generation";
+    };
+
+    cnames = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+      description = "CNAME records pointing to this host";
+      example = [ "web" "api" ];
+    };
+  };
+}