feat(mdns): announce this site's local presence for agent local-discovery

The announcer half of AGENT_LOCAL_DISCOVERY_SPEC.md / MULTI_SITE_SPEC.md
Appendix B -- advertises which public hostnames this site fronts (and at
what local IP) via mDNS, so a theta-agent on the same LAN segment with
prefer_local_directory enabled can skip the relay/WAN path.

Opt-in via THETA_LOCAL_DISCOVERY_HOSTS (comma-separated); no-op if unset,
so this changes nothing for an install that doesn't configure it. Only
ever advertises which hostnames map to which local IP -- no identity/
trust information -- consistent with the hard rule on the listening side
(theta-agent) that local-discovery may change DNS resolution but must
never touch certificate validation.

Verified end-to-end against the real theta-agent Go binary: announce,
discover, apply, and clean revert on disappearance all confirmed working
over real multicast between two containers.

nodejs/bin/www

@@ -19,6 +19,8 @@ require('@simpleworkjs/bao-conf').init({ path: 'jump-host', conf }).then(async (
 	require('../models');
 	const { bootstrapWireguard } = require('../services/wg_bootstrap');
 	await bootstrapWireguard();
+	const { startMdnsAnnounce } = require('../services/mdns_announce');
+	await startMdnsAnnounce();
 
 	const app = require('../app');
 	const middleware = require('../middleware/auth');