Reimplement the streaming chat-display layer

The message-delivery layer was gutted (user::msg_post commented out;
set_context/_send referenced only in comments, never defined), so the
chat could log in but never show or send messages: the stream frame
returned an empty response (502 behind Traefik) and posted messages went
nowhere.

Reimplemented server-streamed chat:
- user: add i_stream_fd (long-lived chat-display connection) + b_stream_ready
  + s_msg buffer. msg_post appends to s_msg and flushes to the fd once the
  initial response is sent (best-effort non-blocking write; buffer on
  EAGAIN; on hard write error drop the reference and mark offline).
  flush_stream/clear_stream helpers; clear_stream on clean()/disconnect.
- reqp (event=stream): build the initial HTTP response (headers, no
  Content-Length so it streams incrementally, + the parsed stream.html
  body), attach this fd to the user, set p_context->p_user, KEEP_ALIVE=yes.
- sock: route KEEP_ALIVE responses through handle_stream_write (sends the
  initial page, then arms a persistent read event) and handle_stream_read
  (drains stray bytes; on EOF/error reaps the context and clears the
  user's stream). The connection stays open for pushed messages.
- context: init p_user=NULL; track i_buf_len across reads (POST body behind
  a proxy arrives in a separate segment).

Verified locally: login -> open stream -> post message -> message appears
in the stream; two users in a room both receive a posted message.

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