I was visiting the UK the other day, went to the supermarket and saw discounts for loyalty card owners. Didn't want to waste any money and a quick Google Image search gave me a barcode I could scan. Then I thought, why don't get charities / clubs / nice people to benefit from this so I built a little static site and now I need people to send me images of cards for others to scan.
https://panelbear.com/ isn’t open source but privacy friendly. I found it on HN (the founder is on here), can’t speak more highly of it. If you are tracking more than one site and want to get a good overview I recommend it.