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/bin/backfill_hashes.rs

@@ -71,7 +71,6 @@ fn main() -> anyhow::Result<()> {
     );
 
     let dao: Arc<Mutex<Box<dyn ExifDao>>> = Arc::new(Mutex::new(Box::new(SqliteExifDao::new())));
-    let ctx = opentelemetry::Context::new();
 
     let mut total_hashed = 0u64;
     let mut total_missing = 0u64;
@@ -84,7 +83,7 @@ fn main() -> anyhow::Result<()> {
         let rows = {
             let mut guard = dao.lock().expect("Unable to lock ExifDao");
             guard
-                .get_rows_missing_hash(&ctx, args.batch_size)
+                .get_rows_missing_hash(args.batch_size)
                 .map_err(|e| anyhow::anyhow!("DB error: {:?}", e))?
         };
         if rows.is_empty() {
@@ -128,7 +127,6 @@ fn main() -> anyhow::Result<()> {
                 match ident {
                     Some(id) => {
                         match guard.backfill_content_hash(
-                            &ctx,
                             *library_id,
                             rel_path,
                             &id.content_hash,