Cookie-backed sessions for Sova (Express express-session-style).
Flash helpers ([Session::flash], [Session::take]) store one-shot values for the next
request (status messages, validation errors, old form input).
Persistence is [SessionStore]: [KvSessionStore], [SqlSessionStore], or
[RedisSessionStore]. Logout others/all:
[SessionExt::logout_other_sessions] / [SessionExt::logout_all_sessions].