Videos: Create fragmented MP4s when transcoding with FFmpeg

[lastzero]

Based on the following recommendations, the value of the FFmpeg -movflags flag should be changed from "+faststart" to "frag_keyframe+empty_moov+default_base_moof+faststart" to ensure properly fragmented and compatible MP4 files that are suitable for streaming with HLS.js:

• https://developer.mozilla.org/en-US/docs/Web/API/Media_Source_Extensions_API/Transcoding_assets_for_MSE#fragmenting
• https://nschlia.github.io/ffmpegfs/html/ffmpeg__profiles_8cc.html
• https://cloudinary.com/glossary/fragmented-mp4
• https://medium.com/@vlad.pbr/in-browser-live-video-using-fragmented-mp4-3aedb600a07e
• https://github.com/video-dev/hls.js?tab=readme-ov-file#features

While streaming has worked before, the hope is that this change will further improve the situation, e.g. when changing the playback position in longer videos that have been transcoded (videos streamed in their original format will not benefit in any way, as they remain untouched to save storage space).

If anyone has any other information or recommendations, now would be a good time to let us know! 🎬