fix: implement random-port discovery for CI E2E backend

When PORT=0, the backend now writes its actual bound port to
data/test/.port. test-env.sh reads that file when TT_TEST_PORT=0
so all targets (test-up, test-reset, test-down) resolve the real URL.
test-up waits for .port to appear before the health-check loop.

backend/src/main.rs

@@ -85,7 +85,14 @@ async fn main() {
     let listener = tokio::net::TcpListener::bind(&addr)
         .await
         .expect("failed to bind");
-    tracing::info!("listening on {}", addr);
+    let actual_addr = listener.local_addr().expect("failed to get local addr");
+    tracing::info!("listening on {}", actual_addr);
+
+    // When started with PORT=0 (random), write actual port so the test harness can discover it
+    if port == "0" {
+        let _ = std::fs::create_dir_all("data/test");
+        let _ = std::fs::write("data/test/.port", actual_addr.port().to_string());
+    }
 
     axum::serve(
         listener,