A 58-year-old woman with rheumatoid arthritis, well-controlled on methotrexate 15mg weekly and folic acid 5mg weekly (taken on a different day), requires analgesia for osteoarthritis pain in her knee. She has mild CKD (eGFR 55 ml/min/1.73m²). According to NICE CG177, which analgesic is generally considered the safest initial option alongside paracetamol?