`KeyError` crash when viewing video (may be due to content warning / age restriction?)

Attempting to view https://www.youtube.com/watch?v=oIWqTIFRlYU in Via (https://via.hypothes.is/https://www.youtube.com/watch?v=oIWqTIFRlYU) results in a 5xx error from our backend: .

Error details from the response:

```json
{
"errors": [
{
"status": 500,
"code": "KeyError",
"title": "Something went wrong",
"detail": "'captions'"
}
]
}
```

This video does have an auto-generated transcript available on YouTube itself. The video is preceded by a content warning (it is about the Israel-Palestine conflict), which might be related.

Reproduce the 5xx response for the YouTube URL and inspect the backend path that processes the video's response, focusing on the missing 'captions' field. Confirm the behavior for videos with auto-generated transcripts and content warnings; done means the URL no longer crashes with KeyError and its available transcript is handled correctly.

Written by the indexing model from the issue text.