Fix web session playback when a duration is not provided

[zmb3]

Prior to Teleport 15, the web UI would download the entire session recording into browser memory before playing it back (crashing the browser tab for long sessions).

Starting with Teleport 15, session playback is streamed to the browser and played back as it is received instead of waiting for the entire session to be available.

A side effect of this change is that the browser needs to know the length of the session in order to render the progress bar during playback. Since the browser starts playing the session before it has received all of it, we started providing the length of the session via a URL query parameter.

Some users have grown accustomed to being able to access session recordings at their original URLs (without the duration query parameter). If you attempt to play recordings from these URLs after upgrading to v15, you'll get an error that the duration is missing.

To fix this, the web UI needs to request the duration of the session before it can begin playing it (unless the duration is provided via the URL). There are two ways we could get this information:

1. By querying Teleport's audit log
2. By reading the recording twice: once to get to the end event and compute the duration, and a second time to actually play it back.

Since we only have a session ID, an audit log query would be inefficient - we have no idea when the session occurred, so we'd have to search from the beginning of time. (This could be resolved by using a UUIDv7 for session IDs, but Teleport uses UUIDv4 today).

For this reason, we elect option 2. This commit creates a new web API endpoint that will fetch a session recording file and scan through it in the same way that streaming is done, but instaed of streaming the data through a websocket it simply reads through to the end to compute the session length. The benefit of this approach is that it will generally be faster than option 1 (unless the session is very long), and it effectively pre-downloads the recording file on the

Note: option 2 is not without its drawbacks - since the web UI is making two requests that both read the session recording, the audit log will show two separate session_recording.access events. This isn't ideal but it is good enough to get playback working again for customers who don't access playbacks by clicking the "Play" button in the UI.

Changelog: Restore the ability to play session recordings in the web UI without specifying the session duration in the URL.

[gzdunek]

I left some minor comments on the JS code.

[public-teleport-github-review-bot]

@zmb3 See the table below for backport results.

Branch	Result
branch/v15	Failed
branch/v16	Failed
branch/v17	Failed