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/library_maintenance.rs

@@ -68,7 +68,6 @@ pub const DEFAULT_SCAN_PAGE_SIZE: i64 = 500;
 /// Caller must already have confirmed the library is online — running
 /// against a Stale library would interpret every row as missing.
 pub fn detect_missing_files_for_library(
-    context: &opentelemetry::Context,
     library: &Library,
     exif_dao: &Arc<Mutex<Box<dyn ExifDao>>>,
     offset: i64,
@@ -77,7 +76,7 @@ pub fn detect_missing_files_for_library(
 ) -> (usize, i64) {
     let rows = {
         let mut dao = exif_dao.lock().expect("exif_dao poisoned");
-        match dao.list_rel_paths_for_library_page(context, library.id, page_size, offset) {
+        match dao.list_rel_paths_for_library_page(library.id, page_size, offset) {
             Ok(r) => r,
             Err(e) => {
                 warn!(
@@ -136,7 +135,7 @@ pub fn detect_missing_files_for_library(
     {
         let mut dao = exif_dao.lock().expect("exif_dao poisoned");
         for rel_path in &to_delete {
-            match dao.delete_exif_by_library(context, library.id, rel_path) {
+            match dao.delete_exif_by_library(library.id, rel_path) {
                 Ok(()) => deleted += 1,
                 Err(e) => warn!(
                     "missing-file scan: delete failed for ({}, {}): {:?}",