ATProto: detect and handle videos that are over Bluesky's 60s limit

[snarfed]

Related to #1348: Bluesky currently only supports short videos, 60 seconds or less. It'd be nice to detect videos that are too long and include a [Video] text suffix, like we used to do, instead of bridging them anyway and having the client show that they failed:

This is technically nontrivial though. I'd need to buffer the entire video in memory, add decoding logic for multiple different formats, who knows what else. I'm unlikely to prioritize this myself, but I'm open to merging a PR if it's reasonable!

[qazmlp]

ffprobe would almost certainly work.
It also takes URLs as input and likely doesn't need to download the entire file.

I'm not sure how well that works with your deployment model, though.

[snarfed]

Sadly yeah, I generally need to avoid command line and filesystem-based tools.