Use faststart instead of fragmented MP4 flags for non-segmented streams.
This fixes seeking and duration metadata in downloaded transcoded files.
When transcoding for download (no segment container), MP4 files now use
the faststart movflag which moves the moov atom to the beginning of the
file. This enables:
- Proper duration metadata for video players
- Accurate time display and seeking
- Better compatibility with standard video players
Fragmented MP4 flags (frag_keyframe+empty_moov+delay_moov) are still
used for adaptive streaming (HLS/DASH) where segment containers are specified.
a2f0eef49b