Integration coverage for `validateSessionFromCookieValue` against real
Postgres: a live secret-backed session resolves to its user, an expired one
and an unknown/forged/empty value return null, and a legacy cuid cookie
resolves only while its row still carries no hashed secret. This is the
hashing + row lookup + expiry the callback's duplicate detection turns on,
which the unit suite stubs out.
Refs #600
