Hotfix/server unexpected freeze

[loclt1011]

• Share one uvicorn log handler and send stderr to a file, not a multilog pipe — the two mechanisms that froze the event loop
• Reopen the log stream after a failed write, so a storage fault degrades logging instead of silencing it until restart
• Return 504 for a timed-out backend and 502 for an unreachable one, inside a ladder that shrinks inward, with one request id carried end to end
• Verify the stop released the port, refuse a second instance over a running one's logs, and SIGKILL a server that stays bound but stops answering /livez
• Reuse one pooled httpx client instead of building one per request, which stranded 433 CLOSE-WAIT sockets