Proposal: Add watch/dwell time to sendInteractions

[kaytwo]

As per TikTok:

A strong indicator of interest, such as whether a user finishes watching a longer video from beginning to end, would receive greater weight than a weak indicator, such as whether the video's viewer and creator are both in the same country.

When creating algorithmic video feeds, watch time will be an important signal to send back to the feed creator. The design decisions/options seem to be what type of value to send back. I could see an argument for:

1. A percentage (either int 0..100 or float 0..1) of the entire video watched. This could work for audio posts as well if they were added. It wouldn't capture scrubbing/rewatching, nor would it make sense for live feeds if they were added.
2. A number of milliseconds watched total, reported when the app/feed loses focus or the user swipes to another post.
3. A coarser quantization event that represents something like "watched 5% of the video", and thus gets sent back 20 times if the user watches the whole video, or 30 if they watch half of it over again. Doesn't feel like a great idea, but would be the easiest to add to the schema (just another event type). Due to the way that events are batched, this shouldn't be particularly onerous perf wise.

IMO #2 is the right way to go - happy to write up a PR for the lexicon if this feature is desired. Could probably add it to the video post view in social-app too.

P.S. I know that these signals are currently only sent for the Bluesky-provided trending videos + discover feeds, but (especially if video feeds become particularly popular on Bluesky) it will be an important feature to add.