fix: switch to ThreadingHTTPServer for concurrent request handling

gateway/server.py

@@ -8,9 +8,7 @@ This file is thin routing only. All logic lives in submodules.
 
 import json
 from datetime import datetime
-from http.server import BaseHTTPRequestHandler
-from socketserver import ThreadingMixIn
-from http.server import HTTPServer
+from http.server import ThreadingHTTPServer, BaseHTTPRequestHandler
 
 from config import (
     PORT, TRIPS_API_TOKEN, KINDLE_EMAIL_1, KINDLE_EMAIL_2,
@@ -339,9 +337,6 @@ def main():
     print(f"[Gateway] Services: {_proxy_module.SERVICE_MAP}")
     print(f"[Gateway] Listening on port {PORT}")
 
-    class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
-        daemon_threads = True
-
     server = ThreadingHTTPServer(("0.0.0.0", PORT), GatewayHandler)
     server.serve_forever()