A 75-year-old resident of a care home develops confusion, pyrexia (38.2°C), and increased urinary frequency. A urine dipstick is positive for leucocytes and nitrites. She has a history of dementia and recurrent UTIs. According to NICE NG109, what is the recommended first-line empirical antibiotic for a suspected UTI in a woman over 65 years old residing in a care home (assuming no recent antibiotic use or resistance history)?