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/thumbnails.rs

@@ -14,19 +14,15 @@ use image::GenericImageView;
 use image::codecs::jpeg::JpegEncoder;
 use lazy_static::lazy_static;
 use log::{debug, error, info, warn};
-use opentelemetry::{
-    KeyValue,
-    trace::{Span, TraceContextExt, Tracer},
-};
 use prometheus::IntGauge;
 use rayon::prelude::*;
+use tracing_opentelemetry::OpenTelemetrySpanExt;
 use walkdir::DirEntry;
 
 use crate::content_hash;
 use crate::exif;
 use crate::file_types;
 use crate::libraries;
-use crate::otel::global_tracer;
 use crate::video::actors::{generate_image_thumbnail_ffmpeg, generate_video_thumbnail};
 
 /// Maximum long-edge size (px) for the large preview tier. Tuned to look
@@ -327,9 +323,9 @@ fn generate_xlarge_preview_ffmpeg(
     Ok(())
 }
 
+#[tracing::instrument(name = "creating thumbnails", skip_all)]
 pub fn create_thumbnails(libs: &[libraries::Library], excluded_dirs: &[String]) {
-    let tracer = global_tracer();
-    let span = tracer.start("creating thumbnails");
+    let span = tracing::Span::current();
 
     let thumbs = &dotenv::var("THUMBNAILS").expect("THUMBNAILS not defined");
     let thumbnail_directory: &Path = Path::new(thumbs);
@@ -387,16 +383,12 @@ pub fn create_thumbnails(libs: &[libraries::Library], excluded_dirs: &[String])
                 }
 
                 if is_video(&entry) {
-                    let mut video_span = tracer.start_with_context(
-                        "generate_video_thumbnail",
-                        &opentelemetry::Context::new()
-                            .with_remote_span_context(span.span_context().clone()),
-                    );
-                    video_span.set_attributes(vec![
-                        KeyValue::new("type", "video"),
-                        KeyValue::new("file-name", thumb_path.display().to_string()),
-                        KeyValue::new("library", lib.name.clone()),
-                    ]);
+                    // Explicit parent: this closure runs on rayon worker
+                    // threads, where the ambient span isn't current.
+                    let video_span = tracing::info_span!(parent: &span, "generate_video_thumbnail");
+                    video_span.set_attribute("type", "video");
+                    video_span.set_attribute("file-name", thumb_path.display().to_string());
+                    video_span.set_attribute("library", lib.name.clone());
 
                     debug!("Generating video thumbnail: {:?}", thumb_path);
                     if let Err(e) = generate_video_thumbnail(src, &thumb_path) {
@@ -409,7 +401,7 @@ pub fn create_thumbnails(libs: &[libraries::Library], excluded_dirs: &[String])
                             warn!("Failed to write sentinel {:?}: {}", sentinel, se);
                         }
                     }
-                    video_span.end();
+                    drop(video_span);
                 } else if is_image(&entry) {
                     match generate_image_thumbnail(src, &thumb_path) {
                         Ok(_) => info!("Saved thumbnail: {:?}", thumb_path),