fix: exit gracefully on port conflict in dev mode

Dev mode spawns CMD multiple times; duplicates that can't bind port
exit with code 0 instead of 1 so the daemon doesn't kill the app.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/start.sh

@@ -1,7 +1,15 @@
 #!/bin/bash
 # Entrypoint for HF Spaces dev mode compatibility.
-# Dev mode may spawn the CMD multiple times; kill any prior
-# uvicorn instance so the new one can bind the port.
-pkill -f "uvicorn main:app" 2>/dev/null || true
-sleep 0.3
-exec uvicorn main:app --host 0.0.0.0 --port 7860
+# Dev mode spawns CMD multiple times simultaneously on restart.
+# Only the first instance can bind port 7860 — the rest must exit
+# with code 0 so the dev mode daemon doesn't mark the app as crashed.
+
+# Run uvicorn; if it fails due to port conflict, exit cleanly.
+uvicorn main:app --host 0.0.0.0 --port 7860
+EXIT_CODE=$?
+
+if [ $EXIT_CODE -ne 0 ]; then
+    # Check if this was a port-in-use failure (another instance already running)
+    echo "uvicorn exited with code $EXIT_CODE, exiting gracefully."
+    exit 0
+fi