Replace NotImplementedError stubs in AuthenticationService with working
implementations: createSession signs a HS256 JWT using Payload's instance
secret, validateSession verifies and decodes the token then looks up the
user, invalidateSession returns a blank cookie with maxAge 0. No external
JWT dependency — uses Node crypto HMAC directly.
Also clarify withAudit/withAnalytics comments: the wrappers intentionally
delegate recording to the use case body (only it knows which fields to
extract), so the TODO was misleading.
