Fix runtime crashes: logd recursion, trim OOB, sock SO_REUSEADDR/accept, md5 session

Multiple latent bugs made the chat crash on real use (login POST segfaulted):

- logd::flush: when a log file can't be opened, log the error to stderr and
  exit(1). Previously it called wrap::system_message, which routes back through
  LOGD->log_simple_line->flush on the same failing logd -> infinite recursion
  -> stack overflow (SIGSEGV). Bitten in k8s where an emptyDir on /app/log
  hides the image's /app/log/rooms, so the room log open failed on login.
- tool::trim: rewrite the right-trim; the original did s_str[s_str.size()]
  (out-of-bounds under _GLIBCXX_ASSERTIONS / UB) and erased at i_pos=size.
- sock::_make_server_socket: move SO_REUSEADDR setsockopt BEFORE bind so a
  quick container restart rebinds port 2000 (was EADDRINUSE -> fallback to
  2001, which the k8s Service doesn't target -> 502).
- sock::process_request: init accept() addrlen and bail on any accept error
  (not just EAGAIN/EINTR) so we never proceed with fd=-1 (EBADF).
- chat.session.md5hash=false at runtime: the md5 session-id path does
  s_hash.substr(s_ret.find(s_salt)+salt.len()+3); the default salt has chars
  not in chat.session.validchars so find() returns npos and the substr/append
  corrupts the heap and segfaults on login. Overridden via -o in the image CMD.
- docker-entrypoint.sh: mkdir -p /app/log/rooms at startup (volume mount hides
  the image's copy).

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