MP4 download details now returned as a structured list

The mp4Support field now returns a structured list instead of a single string. Each entry describes one downloadable file with its type, status, height, width, and ext.

Before:

1
"mp4Support": "capped_4k"

After:

1
"mp4Support": [
2
  { "type": "capped_4k", "status": "ready", "height": 1080, "width": 1920, "ext": "mp4" }
3
]