POST /api/auth/theia-check validates the NextAuth session cookie
forwarded by Traefik. Returns 200 for authenticated users with
X-Auth-User/Email/Name headers, or 302 redirect to /auth for
unauthenticated requests — preserving the original Theia URL as
callbackUrl so users land back in the IDE after login.
Co-authored-by: Cursor <cursoragent@cursor.com>
