fix: run nginx as subprocess (not exec) to keep sync threads alive

os.execvp replaces the Python process, killing all daemon threads
(sync, heartbeat, log streamer). Run nginx as Popen instead and
wait for it, keeping Python as PID 1 with threads running.

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

entrypoint.py

@@ -435,9 +435,17 @@ def main():
     log(f"  SSE:     https://<space>.hf.space/runlog/stream")
     log("========================================")
 
+    # Start nginx as subprocess (NOT exec — we need threads to stay alive)
+    log("starting nginx on 0.0.0.0:7860 ...")
+    nginx_proc = subprocess.Popen(
+        ["nginx", "-c", "/etc/nginx/nginx.conf", "-g", "daemon off;"]
+    )
+    log(f"[  OK  ] nginx PID={nginx_proc.pid}")
+
     # Final save on SIGTERM
     def on_sigterm(sig, frame):
         log(f"signal {sig} — final save ...")
+        nginx_proc.terminate()
         try:
             save_and_upload()
         except Exception as e:
@@ -447,9 +455,8 @@ def main():
     signal.signal(signal.SIGTERM, on_sigterm)
     signal.signal(signal.SIGINT, on_sigterm)
 
-    # exec nginx — becomes PID 1, opens port 7860
-    log("starting nginx on 0.0.0.0:7860 ...")
-    os.execvp("nginx", ["nginx", "-c", "/etc/nginx/nginx.conf", "-g", "daemon off;"])
+    # Wait for nginx to exit (keeps PID 1 = Python, threads alive)
+    nginx_proc.wait()
 
 
 if __name__ == "__main__":