Replaces the in-memory studentSessions/adminSessions Maps (Phase 2B — gone on every
restart) with a sessions table. Browser holds an opaque CSPRNG bearer token; only its
SHA-256 hash is ever stored server-side, same principle as password hashing. Each session
has an absolute expiry (fixed at creation) and a sliding idle expiry (pushed forward on
every authenticated request, capped at the absolute one) — one atomic UPDATE...RETURNING
per resolve, no read-then-write race. Student and instructor sessions stay fully separate
token namespaces, matching Phase 2B's cookie-name separation.
cookies.js centralizes Set-Cookie construction so the Secure attribute (COOKIE_SECURE=1,
for real HTTPS deployments) is applied consistently instead of hand-built per call site.
