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/clip_search.rs

@@ -186,11 +186,9 @@ pub async fn score_photos(
     // before scoring. The caller-supplied `model_version` (or the live
     // engine's) forces a strict join so a mid-flight model swap can't mix
     // geometries.
-    let ctx = opentelemetry::Context::current();
     let rows: Vec<(String, Vec<u8>)> = {
         let mut dao = exif_dao.lock().expect("exif dao");
         dao.list_clip_index(
-            &ctx,
             library_ids,
             model_version.or(Some(&query_resp.model_version)),
         )
@@ -235,19 +233,16 @@ pub fn resolve_hits(
     if scored.is_empty() {
         return Vec::new();
     }
-    let ctx = opentelemetry::Context::current();
     let hashes: Vec<String> = scored.iter().map(|(_, h)| h.clone()).collect();
     let mut dao = exif_dao.lock().expect("exif dao");
-    let path_map = dao
-        .get_rel_paths_for_hashes(&ctx, &hashes)
-        .unwrap_or_else(|e| {
-            log::warn!("clip_search: get_rel_paths_for_hashes failed: {:?}", e);
-            std::collections::HashMap::new()
-        });
+    let path_map = dao.get_rel_paths_for_hashes(&hashes).unwrap_or_else(|e| {
+        log::warn!("clip_search: get_rel_paths_for_hashes failed: {:?}", e);
+        std::collections::HashMap::new()
+    });
 
     let mut results = Vec::with_capacity(scored.len());
     for (score, hash) in scored {
-        let row = match dao.find_by_content_hash(&ctx, hash) {
+        let row = match dao.find_by_content_hash(hash) {
             Ok(Some(r)) => r,
             Ok(None) => continue,
             Err(e) => {