Move the 1-5s commit_session_memory() extraction off the POST /api/session/new
request thread (was blocking + New Chat) into a fire-and-forget daemon thread,
guarded by the existing per-session in-flight serialization.
Data-loss prevention on managed stop:
- server.py installs a SIGTERM handler that requests an orderly stop so
serve_forever()'s finally block runs drain_all_on_shutdown() (the default
SIGTERM disposition terminated without unwinding, losing in-flight commits).
- session_lifecycle: bounded self-unregistering background-commit registry;
a _draining flag so no NEW worker starts once the drain begins (the inline
generation-drain commits any late arrival instead); a 30s overall drain
deadline so a stuck worker cannot hang the stop path forever.
Co-authored-by: luperrypf <luperrypf@users.noreply.github.com>
