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/unified_search.rs

@@ -157,12 +157,10 @@ pub async fn unified_search<TagD: TagDao>(
         Err(msg) => return bad_request(msg),
     };
 
-    let ctx = opentelemetry::Context::current();
-
     // ── 1. Translate the NL query, grounded on the real tag vocabulary ──
     let tag_vocab: Vec<(i32, String)> = {
         let mut dao = tag_dao.lock().expect("tag dao");
-        match dao.get_all_tags(&ctx, None) {
+        match dao.get_all_tags(None) {
             Ok(tags) => tags.into_iter().map(|(_, t)| (t.id, t.name)).collect(),
             Err(e) => {
                 log::warn!("unified_search: get_all_tags failed: {e:?}");
@@ -273,11 +271,7 @@ pub async fn unified_search<TagD: TagDao>(
             None
         } else {
             let mut dao = tag_dao.lock().expect("tag dao");
-            match dao.get_files_with_any_tag_ids(
-                sq.tag_ids.clone(),
-                sq.exclude_tag_ids.clone(),
-                &ctx,
-            ) {
+            match dao.get_files_with_any_tag_ids(sq.tag_ids.clone(), sq.exclude_tag_ids.clone()) {
                 Ok(files) => Some(files.into_iter().map(|f| f.file_name).collect()),
                 Err(e) => {
                     log::warn!("unified_search: tag filter failed: {e:?}");
@@ -298,7 +292,6 @@ pub async fn unified_search<TagD: TagDao>(
         let rows = {
             let mut dao = exif_dao.lock().expect("exif dao");
             dao.query_by_exif(
-                &ctx,
                 None, // scope filtered in-Rust to support multi-library
                 sq.camera_make.as_deref(),
                 sq.camera_model.as_deref(),