Adds sessionKey/signSession/verifySession primitives and
setSessionCookie/clearSessionCookie helpers in a new backend/session.go.
Sessions are derived from API_TOKEN via HMAC-SHA256 with domain
separation (sessionKeyPurpose), so there is no session table and
rotating the token invalidates every outstanding cookie at once.
No routes or handlers yet — that's task 5.
sulthan
7d0eaaef02