fix: add 5s timeout to TURN credential socket

Hung TURN daemon would block dict proxy handler thread indefinitely.
Per Python docs, settimeout() raises TimeoutError on expiry.

chatmaild/src/chatmaild/turnserver.py

@@ -4,6 +4,7 @@ import socket
 
 def turn_credentials() -> str:
     with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as client_socket:
+        client_socket.settimeout(5)
         client_socket.connect("/run/chatmail-turn/turn.socket")
         with client_socket.makefile("rb") as file:
             return file.readline().decode("utf-8").strip()