generalize upstream re-detection into network change watcher

Always detect network changes (LAN IP, upstream, peers) regardless
of upstream config. LAN IP is now tracked in ServerCtx and updated
every 30s — multicast announcements use the current IP instead of
the startup IP. Upstream re-detection still only runs when
auto-detected. Peer flush triggers on any network change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ctx.rs

@@ -33,8 +33,9 @@ pub struct ServerCtx {
     pub lan_peers: Mutex<PeerStore>,
     pub forwarding_rules: Vec<ForwardingRule>,
     pub upstream: Mutex<SocketAddr>,
-    pub upstream_auto: bool, // true = auto-detected, false = explicitly configured
+    pub upstream_auto: bool,
     pub upstream_port: u16,
+    pub lan_ip: Mutex<std::net::Ipv4Addr>,
     pub timeout: Duration,
     pub proxy_tld: String,
     pub proxy_tld_suffix: String, // pre-computed ".{tld}" to avoid per-query allocation