Sounds like a fun project to cut your teeth on.
As said above, avoid mixing mains and diy electronics as far as possible.. IT CAN BE LETHAL.
Quite a few options are open to you though, a light sensitive resistor as suggested above is one. A PIR (passive infra red) or similar motion detector inside the bathroom, a reed switch and magnet pair mounted on the loo lid or on the loo door?
For more sophistication how about personalised music depending on who is in the loo?? if everyone carries a phone 24/7 (Or if you can convince them to carry a £shop ble tracker) you could use ble to id which phone/tracker is in the b/room (closest to the pi) Or a load cell under the seat could indicate who is in the b/room based on their mass.
fwiw my own experiments with home automation and using a Pi-0-W as a headless/wireless system controller is based on re-using and in some cases reprogramming off the shelf 'smart' consumer retail devices.
For example to satisfy your requirements for less than a £20 cost i would use any battery operated alarm system PIR which broadcasts a 433mhz RF signal (£5+) and use a sonoff RF bridge reprogrammed with tasmota firmware(£8+)..
the rfbridge is a sonoff brand device for use with sonoff brand smart switches But its chinese firmware can be overwritten with open source code from Theo Arends https://github.com/arendst/Sonoff-Tasmota/wiki
A live, dynamic project with regular updates and support fixes.
A Tasmota flashed RF bridge will sit and decode any and all 433mhz signals it can hear and generate a wifi mqtt topic/payload message for each making the signals visible to any device on the same network. so you can simply subscribe to the relevant mqtt topic in your control app /system and whenever the signal you expect is broadcast you will know..
http://www.steves-internet-guide.com/py ... subscribe/
The s/w I use on my Pi for home automation is Mosquitto Mqtt Broker, and for implementing my logic and control Node Red. all free...
Thats just my take tho, .. Lots of other ways and options.. have fun..