Default --port to 0 for OS-assigned ephemeral port (ticket 019)

Reads back the actual bound port via listener.local_addr() since
binding to 0 means the requested and real ports differ.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/main.rs

@@ -25,8 +25,9 @@ use settings::Settings;
 /// Observability dashboard from terminal output.
 #[derive(Parser, Debug)]
 struct Args {
-    /// Port to serve the API and webapp on.
+    /// Port to serve the API and webapp on. Defaults to 0, letting the OS assign
-    #[arg(long, default_value_t = 8420)]
+    /// an ephemeral free port.
+    #[arg(long, default_value_t = 0)]
     port: u16,
 
     /// Directory containing the built webapp (index.html + assets).
@@ -91,10 +92,14 @@ async fn main() {
     let listener = TcpListener::bind(("0.0.0.0", args.port))
         .await
         .expect("failed to bind listener");
+    let bound_port = listener
+        .local_addr()
+        .expect("failed to read back bound address")
+        .port();
 
     eprintln!(
         "ottyl serving on http://localhost:{} (reading logs from stdin)",
-        args.port
+        bound_port
     );
 
     axum::serve(listener, app)