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/watcher.rs

@@ -125,6 +125,7 @@ pub fn cleanup_orphaned_playlists(
             }
 
             info!("Running orphaned HLS cleanup");
+            let _pass = tracing::info_span!("watcher.playlist_cleanup").entered();
             let start = std::time::Instant::now();
 
             // Snapshot every live content_hash currently in image_exif.
@@ -418,6 +419,15 @@ pub fn watch_files(
 
             let is_full_scan = since_last_full.as_secs() >= full_interval_secs;
 
+            // Root span per tick: this loop runs on a plain std thread, so a
+            // synchronous `entered()` guard is the right shape. Everything the
+            // tick does (probes, drains, ingest, maintenance) nests under it.
+            let _tick = tracing::info_span!(
+                "watcher.tick",
+                scan = if is_full_scan { "full" } else { "quick" },
+            )
+            .entered();
+
             // Fresh snapshot per tick — picks up PATCH /libraries/{id}
             // mutations to `enabled` / `excluded_dirs` without restart.
             let libs: Vec<libraries::Library> =
@@ -436,6 +446,12 @@ pub fn watch_files(
                     );
                     continue;
                 }
+                let _lib_span = tracing::info_span!(
+                    "watcher.library",
+                    library = %lib.name,
+                    library_id = lib.id,
+                )
+                .entered();
 
                 // Availability probe: every tick checks that the
                 // library's mount is reachable, is a directory, is