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You could try https://ntfy.sh/
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Stream
Stream - Scalable APIs for Chat, Feeds, Moderation, & Video. Stream helps developers build engaging apps that scale to millions with performant and flexible Chat, Feeds, Moderation, and Video APIs and SDKs powered by a global edge network and enterprise-grade infrastructure.
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A bit unrelated and it's not open source, but if you have a lot of media files and the file naming conventions bother you, I recommend Filebot to format and rename everything, it's a godsend. I think it costs money now, but if you do a lot of turning it's worth it.
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Healthchecks
Open-source cron job and background task monitoring service, written in Python & Django
If you're really interested in something like that, it's pretty easy. Just use something like healthchecks.io, cronitor or cronhub.