Background work has no request parent, so each job now opens a named
tracing root and carries it into its fan-out:
- watcher: `watcher.tick` (scan=quick|full) entered per tick on the
watcher's std thread, `watcher.library` per library, and
`watcher.playlist_cleanup` per orphaned-HLS pass. face_watch /
clip_watch carry the ambient span into every per-candidate
tokio::spawn and give each call its own `face_watch.detect` /
`clip_watch.encode` span.
- thumbnails: `creating thumbnails` via #[instrument]; per-video spans on
rayon workers name their parent explicitly.
- video actors: `playlistgenerator.generate_playlist` and
`preview_clip.generate` are tracing roots that instrument their futures.
- reels: `reel.job` (created before the spawn, so on-demand jobs stay in
the request trace), `reel.pregen_batch`, `reel.cache_sweep`.
- RefreshThumbnailsMessage and the dead-code GIF job move to tracing.
No raw OpenTelemetry span creation is left outside otel.rs;
`global_tracer` is removed.
tests/otel_background.rs pins the std-thread tick -> block_on -> spawn
shape and the rayon explicit-parent shape.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
