Backing store for OAuth state tokens. Default impl keeps them in memory
(fine for tests + dev); the runtime swaps in a SQLite-backed impl so a
restart in the middle of an OAuth handshake doesn’t leave the user with
“invalid state” on the callback. Same pattern as SessionBackend.
Pluggable storage backend for sessions. The default is in-memory; apps
deploying for real should supply a persistent backend (e.g. SQLite or
Redis) so users don’t log out on server restart.