fix(#1356): replace HTTPServer with ThreadingHTTPServer in multi-user bridge

The single-threaded HTTPServer queues requests sequentially, causing
60% timeout under 10 concurrent users (30s timeout, 7.8s avg).

Fix: use ThreadingHTTPServer (already defined in multi_user_bridge.py)
for thread-per-request concurrency.

- multi_user_bridge.py: line 2883 HTTPServer -> ThreadingHTTPServer
- world/multi_user_bridge.py: add ThreadingMixIn import + class, fix server init

Refs #1356

world/multi_user_bridge.py

@@ -34,10 +34,9 @@ from typing import Optional
 
 
 class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
-    """Thread-per-request HTTP server for concurrent user handling."""
+    """Thread-per-request HTTP server."""
     daemon_threads = True
 
-
 # ── Configuration ──────────────────────────────────────────────────────
 
 BRIDGE_PORT = int(os.environ.get('TIMMY_BRIDGE_PORT', 4004))