fix(system): hostname rename updates /etc/hosts and re-announces mDNS

server.set-name ran hostnamectl but left Debian's 127.0.1.1 line on the
old name (breaking sudo's 'unable to resolve host' and hostname -f) and
waited on avahi to notice the change by itself. Sync /etc/hosts and
avahi-set-host-name (fallback: try-restart avahi-daemon) right after a
successful rename, so http(s)://<hostname>.local works immediately —
best-effort, never blocks the rename.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/api/rpc/system/handlers.rs

@@ -53,6 +53,7 @@ impl RpcHandler {
         // hit a hostname-mismatch warning on top of the usual self-signed one
         // the moment a node is renamed.
         if hostname_updated {
+            sync_hostname_side_effects(&hostname).await;
             if let Err(e) = regenerate_tls_cert(&hostname).await {
                 warn!(hostname = %hostname, "TLS cert regen after rename failed: {}", e);
             }
@@ -375,6 +376,46 @@ pub(super) fn hostname_from_server_name(name: &str) -> String {
     }
 }
 
+/// Post-rename side effects that keep the OS consistent with the new
+/// hostname — all best-effort, the rename itself has already succeeded.
+/// Debian resolves the local hostname via the 127.0.1.1 line in /etc/hosts;
+/// leaving the old name there breaks `sudo` ("unable to resolve host") and
+/// `hostname -f`. And while avahi eventually follows the kernel hostname,
+/// re-announcing immediately makes http(s)://<hostname>.local links work
+/// right after the rename instead of minutes later.
+async fn sync_hostname_side_effects(hostname: &str) {
+    // hostname_from_server_name guarantees [a-z0-9-], safe to interpolate.
+    let script = format!(
+        "if grep -q '^127\\.0\\.1\\.1' /etc/hosts; then sed -i 's/^127\\.0\\.1\\.1.*/127.0.1.1\\t{h}/' /etc/hosts; else printf '127.0.1.1\\t{h}\\n' >> /etc/hosts; fi",
+        h = hostname
+    );
+    match tokio::process::Command::new("/usr/bin/sudo")
+        .args(["-n", "/bin/sh", "-c", &script])
+        .output()
+        .await
+    {
+        Ok(o) if o.status.success() => {}
+        Ok(o) => warn!(
+            "/etc/hosts hostname sync failed: {}",
+            String::from_utf8_lossy(&o.stderr).trim()
+        ),
+        Err(e) => warn!("/etc/hosts hostname sync failed: {}", e),
+    }
+
+    let republished = tokio::process::Command::new("/usr/bin/sudo")
+        .args(["-n", "/usr/bin/avahi-set-host-name", hostname])
+        .output()
+        .await
+        .map(|o| o.status.success())
+        .unwrap_or(false);
+    if !republished {
+        let _ = tokio::process::Command::new("/usr/bin/sudo")
+            .args(["-n", "/usr/bin/systemctl", "try-restart", "avahi-daemon"])
+            .output()
+            .await;
+    }
+}
+
 async fn set_system_hostname(hostname: &str) -> Result<()> {
     let output = tokio::process::Command::new("/usr/bin/sudo")
         .args(["-n", "/usr/bin/hostnamectl", "set-hostname", hostname])