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/lan.rs

@@ -113,7 +113,7 @@ pub async fn start_lan_discovery(ctx: Arc<ServerCtx>, config: &LanConfig) {
             .as_nanos() as u64;
         pid ^ ts
     };
-    let local_ip = detect_lan_ip().unwrap_or(Ipv4Addr::LOCALHOST);
+    let local_ip = *ctx.lan_ip.lock().unwrap();
     info!(
         "LAN discovery on {}:{}, local IP {}, instance {:016x}",
         multicast_group, port, local_ip, instance_id
@@ -142,7 +142,6 @@ pub async fn start_lan_discovery(ctx: Arc<ServerCtx>, config: &LanConfig) {
     // Spawn sender
     let sender_ctx = Arc::clone(&ctx);
     let sender_socket = Arc::clone(&socket);
-    let local_ip_str = local_ip.to_string();
     let dest = SocketAddr::new(IpAddr::V4(multicast_group), port);
     tokio::spawn(async move {
         let mut ticker = tokio::time::interval(interval);
@@ -162,9 +161,10 @@ pub async fn start_lan_discovery(ctx: Arc<ServerCtx>, config: &LanConfig) {
             if services.is_empty() {
                 continue;
             }
+            let current_ip = sender_ctx.lan_ip.lock().unwrap().to_string();
             let announcement = Announcement {
                 instance_id,
-                host: local_ip_str.clone(),
+                host: current_ip,
                 services,
             };
             if let Ok(json) = serde_json::to_vec(&announcement) {