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/video/actors.rs

@@ -806,10 +806,8 @@ impl Handler<GeneratePreviewClipMessage> for PreviewClipGenerator {
 
             // Update status to processing
             {
-                let otel_ctx = opentelemetry::Context::current();
                 let mut dao = preview_dao.lock().expect("Unable to lock PreviewDao");
-                let _ =
-                    dao.update_status(&otel_ctx, &relative_path, "processing", None, None, None);
+                let _ = dao.update_status(&relative_path, "processing", None, None, None);
             }
 
             // Compute output path: join preview_clips_dir with relative path, change ext to .mp4
@@ -827,10 +825,8 @@ impl Handler<GeneratePreviewClipMessage> for PreviewClipGenerator {
                             "Preview clip complete for '{}' ({:.1}s, {} bytes)",
                             relative_path_owned, duration, size
                         );
-                        let otel_ctx = opentelemetry::Context::current();
                         let mut dao = preview_dao.lock().expect("Unable to lock PreviewDao");
                         let _ = dao.update_status(
-                            &otel_ctx,
                             &relative_path_owned,
                             "complete",
                             Some(duration as f32),
@@ -843,10 +839,8 @@ impl Handler<GeneratePreviewClipMessage> for PreviewClipGenerator {
                             "Failed to generate preview clip for '{}': {}",
                             relative_path_owned, e
                         );
-                        let otel_ctx = opentelemetry::Context::current();
                         let mut dao = preview_dao.lock().expect("Unable to lock PreviewDao");
                         let _ = dao.update_status(
-                            &otel_ctx,
                             &relative_path_owned,
                             "failed",
                             None,