Clients hitting the public API directly (e.g. township via nginx) disconnected
during long waits: the direct proxy() path acquired the front queue slot silently
and awaited the engine's first byte (model load) with no output. The broker path
already kept alive; the direct path now does too.

For a STREAMING inference request, commit to a 200 text/event-stream up front and
emit keepalive while acquiring the queue slot and during the engine's model load /
not-ready retries, then relay the real stream (token-counting for the Tasks page),
ending cleanly if the engine dies mid-flight.

Configurable mode (models.wait_status_mode, global default 'invisible'; per-model
override via the models.json entry):
  * invisible — empty-content SSE chunk + x_queue_info (holds the connection; no
                content pollution)
  * visible   — short visible status text (appears in the content)
  * silent    — nothing (legacy path)
When thinking is enabled the keepalive goes on the reasoning channel instead
(no pollution), unless mode is silent.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce