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>

tests/otel_db_spans.rs

@@ -14,8 +14,8 @@
 //! One process-global subscriber per test binary, so this file holds a
 //! single test.
 
-use opentelemetry::trace::{Status, TracerProvider};
 use opentelemetry::Value;
+use opentelemetry::trace::{Status, TracerProvider};
 use opentelemetry_sdk::trace::{InMemorySpanExporter, SdkTracerProvider, SpanData};
 use tracing::Instrument;
 use tracing_opentelemetry::OpenTelemetrySpanExt;
@@ -85,7 +85,10 @@ fn db_spans_nest_under_ambient_span() {
     let del = find(&spans, "db.delete.remove_tag");
 
     for (span, qt, op) in [(get, "query", "get_tags"), (del, "delete", "remove_tag")] {
-        assert_eq!(span.span_context.trace_id(), handler.span_context.trace_id());
+        assert_eq!(
+            span.span_context.trace_id(),
+            handler.span_context.trace_id()
+        );
         assert_eq!(
             span.parent_span_id,
             handler.span_context.span_id(),