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/testhelpers.rs

@@ -93,12 +93,7 @@ impl TestPreviewDao {
 }
 
 impl PreviewDao for TestPreviewDao {
-    fn insert_preview(
-        &mut self,
-        _context: &opentelemetry::Context,
-        file_path_val: &str,
-        status_val: &str,
-    ) -> Result<(), DbError> {
+    fn insert_preview(&mut self, file_path_val: &str, status_val: &str) -> Result<(), DbError> {
         let mut clips = self.clips.lock().unwrap();
         // insert_or_ignore semantics: skip if key already exists
         if clips.contains_key(file_path_val) {
@@ -126,7 +121,6 @@ impl PreviewDao for TestPreviewDao {
 
     fn update_status(
         &mut self,
-        _context: &opentelemetry::Context,
         file_path_val: &str,
         status_val: &str,
         duration: Option<f32>,
@@ -149,17 +143,12 @@ impl PreviewDao for TestPreviewDao {
         }
     }
 
-    fn get_preview(
-        &mut self,
-        _context: &opentelemetry::Context,
-        file_path_val: &str,
-    ) -> Result<Option<VideoPreviewClip>, DbError> {
+    fn get_preview(&mut self, file_path_val: &str) -> Result<Option<VideoPreviewClip>, DbError> {
         Ok(self.clips.lock().unwrap().get(file_path_val).cloned())
     }
 
     fn get_previews_batch(
         &mut self,
-        _context: &opentelemetry::Context,
         file_paths: &[String],
     ) -> Result<Vec<VideoPreviewClip>, DbError> {
         let clips = self.clips.lock().unwrap();
@@ -169,11 +158,7 @@ impl PreviewDao for TestPreviewDao {
             .collect())
     }
 
-    fn get_by_status(
-        &mut self,
-        _context: &opentelemetry::Context,
-        status_val: &str,
-    ) -> Result<Vec<VideoPreviewClip>, DbError> {
+    fn get_by_status(&mut self, status_val: &str) -> Result<Vec<VideoPreviewClip>, DbError> {
         let clips = self.clips.lock().unwrap();
         Ok(clips
             .values()