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/installer/setup.nix

@@ -391,7 +391,7 @@ let
                 + (if $v6 == "y" then { ipv6: { pd: true } } else {} end))
               }
             }
-            + (if $domain == "" then {} else { dns: { domain: $domain } } end))
+            + (if $domain == "" then {} else { dns: { domain: $domain, dynamicLeases: true } } end))
           }' > "$config/settings.json"
 
         if [ "$wan_type" = pppoe ]; then