Chapter markers don't round-trip correctly with transitions

[ekimekim]

Unfortunately I didn't fully debug this issue when I experienced it, but while editing a video with a large number of short (0.5s) transitions and chapter markers at the start of each range, I had two issues:

• The chapter markers moved from the start of the range to the end of the previous (this is annoying but not incorrect - those two time ranges overlap. This is a UX fix.)
• Over a few round trips, the chapter markers seemed to slowly move backwards in time, until they were over 30sec out of sync. This is a correctness issue.

There is likely a bug when parsing the chapter markers from video times back into editor times.

[ekimekim]

For a concrete reproduction, see event with id b635ebbf-c9eb-42a7-b8ef-1a3fdfdc07a6 from DB2024. When loaded in the editor, all chapters are exactly 0.5s earlier than they should be.