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/clip_watch.rs

@@ -29,6 +29,7 @@ use log::{debug, info, warn};
 use std::path::Path;
 use std::sync::{Arc, Mutex};
 use tokio::sync::Semaphore;
+use tracing::Instrument;
 
 /// One file the watcher would like to CLIP-encode. Built from the DAO
 /// `list_clip_unencoded_candidates` result — needs the `content_hash`
@@ -90,6 +91,9 @@ pub fn run_clip_encoding_pass(
 
     let library_id = library.id;
     let library_root = library.root_path.clone();
+    // The detect tasks run on this runtime's worker threads; carry the
+    // watcher's span into each so their spans stay in the tick's trace.
+    let parent = tracing::Span::current();
     rt.block_on(async move {
         let sem = Arc::new(Semaphore::new(concurrency));
         let mut handles = Vec::with_capacity(filtered.len());
@@ -98,10 +102,14 @@ pub fn run_clip_encoding_pass(
             let clip_client = clip_client.clone();
             let exif_dao = exif_dao.clone();
             let library_root = library_root.clone();
-            handles.push(tokio::spawn(async move {
-                let _permit = permit_sem.acquire().await.expect("clip semaphore");
-                process_one(library_id, &library_root, cand, &clip_client, exif_dao).await;
-            }));
+            let task_span = parent.clone();
+            handles.push(tokio::spawn(
+                async move {
+                    let _permit = permit_sem.acquire().await.expect("clip semaphore");
+                    process_one(library_id, &library_root, cand, &clip_client, exif_dao).await;
+                }
+                .instrument(task_span),
+            ));
         }
         for h in handles {
             let _ = h.await;
@@ -109,6 +117,7 @@ pub fn run_clip_encoding_pass(
     });
 }
 
+#[tracing::instrument(name = "clip_watch.encode", skip_all, fields(rel_path = %cand.rel_path))]
 async fn process_one(
     library_id: i32,
     library_root: &str,