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/reels/selector.rs

@@ -279,7 +279,6 @@ pub fn form_beats(
 pub fn resolve(
     app_state: &AppState,
     exif_dao: &Mutex<Box<dyn ExifDao>>,
-    span_context: &opentelemetry::Context,
     selector: &ReelSelector,
 ) -> Result<(Vec<PlannedBeat>, ReelMeta), String> {
     match selector {
@@ -294,7 +293,6 @@ pub fn resolve(
             let items = memories::gather_memory_items(
                 app_state,
                 exif_dao,
-                span_context,
                 *span,
                 zone,
                 library.as_deref(),
@@ -344,7 +342,6 @@ fn distinct_years(items: &[memories::MemoryItem], zone: chrono_tz::Tz) -> Vec<i3
 pub fn enrich(
     insight_dao: &Mutex<Box<dyn InsightDao>>,
     exif_dao: &Mutex<Box<dyn ExifDao>>,
-    span_context: &opentelemetry::Context,
     beats: &mut [PlannedBeat],
 ) {
     let Ok(mut insight_dao) = insight_dao.lock() else {
@@ -360,13 +357,13 @@ pub fn enrich(
             }
             None => continue,
         };
-        if let Ok(Some(insight)) = insight_dao.get_insight(span_context, &rel_path) {
+        if let Ok(Some(insight)) = insight_dao.get_insight(&rel_path) {
             beat.insight_title = Some(insight.title);
             beat.insight_summary = Some(insight.summary);
         }
         // Enrich GPS from EXIF when the lead media is a photo.
         if let Some(SegmentMedia::Photo { .. }) = beat.media.first()
-            && let Ok(Some(exif)) = exif_dao.get_exif(span_context, &rel_path)
+            && let Ok(Some(exif)) = exif_dao.get_exif(&rel_path)
             && let (Some(lat), Some(lon)) = (exif.gps_latitude, exif.gps_longitude)
         {
             beat.gps = Some((lat as f64, lon as f64));