Log 500-return paths in PATCH /image/faces/{id}

The four 500-return paths in update_face_handler returned e.to_string()
in the body but never logged. When a face PATCH failed with a 16-byte
body and no log entry, the cause (SQLITE_BUSY from cross-DAO writer
contention exhausting the 5s busy_timeout) was invisible. Surface the
full anyhow chain via {:#} on each path so the diesel cause is in the
log even when the response body only shows the top-level context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/faces.rs

@@ -2018,12 +2018,19 @@ async fn update_face_handler<D: FaceDao>(
             match dao.get_face(&span_context, id) {
                 Ok(Some(r)) => r,
                 Ok(None) => return HttpResponse::NotFound().finish(),
-                Err(e) => return HttpResponse::InternalServerError().body(e.to_string()),
+                Err(e) => {
+                    warn!("PATCH /image/faces/{}: 500 — get_face failed: {:#}", id, e);
+                    return HttpResponse::InternalServerError().body(e.to_string());
+                }
             }
         };
         let library = match app_state.library_by_id(current.library_id) {
             Some(l) => l.clone(),
             None => {
+                warn!(
+                    "PATCH /image/faces/{}: 500 — face row references unknown library_id {}",
+                    id, current.library_id
+                );
                 return HttpResponse::InternalServerError().body(format!(
                     "face row references unknown library_id {}",
                     current.library_id
@@ -2106,7 +2113,14 @@ async fn update_face_handler<D: FaceDao>(
     let mut dao = face_dao.lock().expect("face dao lock");
     let row = match dao.update_face(&span_context, id, person_patch, bbox_patch, new_embedding) {
         Ok(r) => r,
-        Err(e) => return HttpResponse::InternalServerError().body(e.to_string()),
+        Err(e) => {
+            // The full anyhow chain (`{:#}`) shows the diesel cause behind
+            // the short context string we surface in the response body —
+            // SQLITE_BUSY here usually means another DAO's writer held the
+            // lock past `busy_timeout` (5s), which is invisible in `{}`.
+            warn!("PATCH /image/faces/{}: 500 — update_face failed: {:#}", id, e);
+            return HttpResponse::InternalServerError().body(e.to_string());
+        }
     };
     // Hydrate person_name so the response shape matches GET /image/faces
     // — the carousel overlay does an optimistic replace on this row, and
@@ -2114,7 +2128,13 @@ async fn update_face_handler<D: FaceDao>(
     // VFD label off the bbox even though the assignment didn't change.
     match hydrate_face_with_person(&mut *dao, &span_context, row) {
         Ok(joined) => HttpResponse::Ok().json(joined),
-        Err(e) => HttpResponse::InternalServerError().body(e.to_string()),
+        Err(e) => {
+            warn!(
+                "PATCH /image/faces/{}: 500 — hydrate_face_with_person failed: {:#}",
+                id, e
+            );
+            HttpResponse::InternalServerError().body(e.to_string())
+        }
     }
 }