feat(storage): make MP4 faststart configurable ([storage] mp4_faststart)

[hotkhwan]

Every recorded MP4 segment is finalised with movflags=+faststart, which re-reads the whole segment and re-writes it with the moov atom at the front. On network storage this roughly triples the I/O: measured on a 41-camera site recording ~8.7 MB/s to a single-disk NFS share, the NAS saw ~15 MB/s of writes plus ~13.6 MB/s of reads, and playback reads slowed to 3-8 MB/s.

Add [storage] mp4_faststart (default true, the current behaviour) so a site recording to a NAS or slow HDD can skip the rewrite. Players that use HTTP range requests, including the web UI, play either layout. The option is read from and written back to the ini, exposed in the settings API, and applies to segments opened after a change.

[github-actions]

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[hotkhwan]

Superseded by #633