Tracing: DAO layer + handler spans on tracing, drop all Context params (#141)

trace_db_call no longer takes an OpenTelemetry Context: it opens a
db.<query_type>.<operation> span (same name via otel.name, same
db.query_type / db.operation attrs) on the caller's ambient tracing span
and runs the DAO body inside it. The cx / ctx / context parameter is gone
from every DAO trait method, impl and call site, and from the helpers and
background drains that only threaded it.

The remaining request-handler spans (files, tags, faces, image, video,
favorites, duplicates, memories) move to #[instrument]; the
Context::current_with_span / with_remote_span_context plumbing is gone, as
is extract_context_from_request (the tracing-actix-web root span covers
it). otel::set_span_attributes replaces BoxedSpan::set_attributes.

Background-job raw spans (thumbnails, video actors) are left for #142.

tests/otel_db_spans.rs pins name, attributes, status and nesting (across
an instrumented spawn); otel_traceparent now uses an #[instrument]
handler. CLAUDE.md documents the tracing conventions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/libraries.rs

@@ -492,7 +492,6 @@ pub async fn list_libraries(_claims: Claims, app_state: Data<AppState>) -> impl
         .library_health
         .read()
         .unwrap_or_else(|e| e.into_inner());
-    let context = opentelemetry::Context::new();
     let mut exif_dao = app_state
         .insight_generator
         .exif_dao()
@@ -508,9 +507,7 @@ pub async fn list_libraries(_claims: Claims, app_state: Data<AppState>) -> impl
                 .unwrap_or(LibraryHealth::Online),
             // Live count off the partial index — cheap, and exact even for
             // a stale library the watcher isn't refreshing.
-            date_backfill_remaining: exif_dao
-                .count_rows_needing_utc_backfill(&context, lib.id)
-                .ok(),
+            date_backfill_remaining: exif_dao.count_rows_needing_utc_backfill(lib.id).ok(),
         })
         .collect();
     HttpResponse::Ok().json(LibrariesResponse {