First post so hello everyone and thank you for all the inspiration and knowledge I found here!
Here’s how I made my dumb lock smart.
Step 1 - I placed Fibaro Door/Window sensor inside the cavity in the door frame.
Step 2 - Using two-sided tape I sticked small neodymium magnet to the bolt.
The sensor is battery powered so I didn’t have to do any wiring.
I had to remove the cover of the sensor because otherwise it wouldn’t get through the clearance. That 1 mm made a difference 
I also needed to pad the sensor with foam so that it’s closer to the bolt. Otherwise it wouldn’t trigger the sensor.
It was a bit tricky to align the sensor with the magnet on the bolt. To make it easy I glued two even smaller magnets (1 x 4 mm) on the other side of the foam. Those magnets are weak so they don’t trigger the sensor but they are strong enough to stick to the metal inside the lock cavity. This allows me to secure the sensor in position aligned with the bolt. It will also make it easy to remove the sensor for battery change.
It’s been only 1 day since I got this working so I’m not sure if the double-sided tape will reliably hold the magnet on the bolt. I might have to glue it later.
The magnet on the bolt is N38 neodymium 1 mm thick, 10 mm in diameter. Supposedly it pulls 0,5kg but I never checked. It triggers the sensor reliably.
Sensor https://www.fibaro.com/en/products/door-window-sensor/
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