Publish DHCP clients' names in DNS

router-webui leases-dns reads Kea's leases and publishes <name>.<domain>
with reverse records in the running Unbound (router.dns.dynamicLeases).
Names the configuration publishes are never taken over; a client's name
is cut to its first label and kept inside the local domain; of two
clients the newer lease wins. The DNS page switches it on and shows what
is published and what was skipped; router-setup enables it with a domain.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

modules/router/options.nix

@@ -528,6 +528,21 @@ in
       description = "Like `dns.hosts`, but the name and everything below it answer with these addresses (wildcard).";
     };
 
+    dns.dynamicLeases = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Publish the host names of DHCP clients as `<name>.<dns.domain>` (with
+        reverse records), updated as leases come and go. Needs `dns.domain`.
+
+        A client picks its own name, so only its first label is used, only
+        under the local domain, and a name that is configured here (`dns.hosts`,
+        a reservation, the router itself, a wildcard or a forward zone it falls
+        under) is never taken over. Of two clients claiming one name the newer
+        lease wins. Run by `router-webui leases-dns` (the settings module).
+      '';
+    };
+
     dns.forwardZones = mkOption {
       type = types.attrsOf (types.listOf types.str);
       default = { };