- Server: chapters come from yt-dlp's `chapters` (creator chapters and
description timestamps) whenever a video is resolved, kept in a yt_chapters
table; GET /api/chapters?v= serves them (resolving once if needed, 30-day
cache) so server-cached copies get them too. Never fails the player.
- Player: a video's shared chapters win; with none, YouTube's stand in —
listed as "Chapters from YouTube", read-only (no edit/delete; + Chapter
copies them into an editable list). They drive the existing seek-bar ticks
(classic and Glass Stage), the chapter badge and the chapter list, and are
cached offline with the video's notes.
- Keys: [ previous chapter (restarts the current one when >3 s in), ] next.