4 cases: returns Session with the generated token + supplied
user; createdAt matches injected Clock now; expiresAt is now+7d;
session is findable via SessionRepository->findByToken. fails
red - CreateSession class missing.
9 cases: null/empty/malformed email -> BadRequest; null/empty
password -> BadRequest; unknown email -> Unauthorized; wrong
password -> Unauthorized; valid creds return the User entity;
isAdmin flag survives the auth round-trip. fails red - the
AuthenticateUser class does not exist yet.
