Create Guest Session

Generate a new guest session ID for unauthenticated users. Frontend should store this in localStorage/cookie and send with uploads. Also tracks IP address to prevent abuse (max 3 sessions per IP per day).

Returns: { "session_id": "guest_xxxxx", "max_files": 1, "max_pages": 46, "message": "Guest session created. You can upload 1 file with up to 46 pages." }