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/hls_stats.rs

@@ -117,10 +117,9 @@ pub fn compute_and_publish(
     exif_dao: &Arc<Mutex<Box<dyn ExifDao>>>,
     video_dir: &Path,
 ) -> Vec<HlsLibraryStats> {
-    let ctx = opentelemetry::Context::new();
     let mut out = Vec::with_capacity(libraries.len());
     for lib in libraries {
-        let stats = compute_for_library(&ctx, lib, exif_dao, video_dir);
+        let stats = compute_for_library(lib, exif_dao, video_dir);
         publish_gauges(&stats);
         out.push(stats);
     }
@@ -143,14 +142,13 @@ fn publish_gauges(s: &HlsLibraryStats) {
 }
 
 fn compute_for_library(
-    ctx: &opentelemetry::Context,
     lib: &Library,
     exif_dao: &Arc<Mutex<Box<dyn ExifDao>>>,
     video_dir: &Path,
 ) -> HlsLibraryStats {
     let rows = {
         let mut dao = exif_dao.lock().expect("Unable to lock ExifDao");
-        match dao.list_paths_and_hashes_for_library(ctx, lib.id) {
+        match dao.list_paths_and_hashes_for_library(lib.id) {
             Ok(r) => r,
             Err(e) => {
                 warn!(