Tracing: root spans for background jobs, retire raw OTel spans (#142)

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>

src/files.rs

@@ -30,11 +30,10 @@ use actix_web::{
 use chrono::{DateTime, Utc};
 use log::{debug, error, info, trace, warn};
 use opentelemetry::KeyValue;
-use opentelemetry::trace::{Status, Tracer};
+use opentelemetry::trace::Status;
 
 use crate::data::SortType::NameAsc;
 use crate::error::IntoHttpError;
-use crate::otel::global_tracer;
 use crate::tags::{FileWithTagCount, TagDao};
 use crate::video::actors::StreamActor;
 use path_absolutize::*;
@@ -1373,9 +1372,8 @@ impl Message for RefreshThumbnailsMessage {
 impl Handler<RefreshThumbnailsMessage> for StreamActor {
     type Result = ();
 
+    #[tracing::instrument(name = "RefreshThumbnailsMessage", skip_all)]
     fn handle(&mut self, _msg: RefreshThumbnailsMessage, _ctx: &mut Self::Context) -> Self::Result {
-        let tracer = global_tracer();
-        let _ = tracer.start("RefreshThumbnailsMessage");
         info!("Refreshing thumbnails after upload");
         // The stub in lib.rs is a no-op; the real generation is driven by
         // the file watcher tick in main.rs, which has access to the