Function delete_single_object 

pub async fn delete_single_object(
    client: &Client,
    bucket: &str,
    key: &str,
) -> Result<(), AppError>

Delete a single object from bucket at key.

Used internally by move_object after a successful copy. Exposed pub so task-35 (delete batch) can call it for single-item fallback paths.

Errors

• AppError::AccessDenied — s3:DeleteObject permission denied.
• AppError::NotFound — bucket does not exist.
• AppError::Network — any other SDK or transport error.

Note: S3 DeleteObject on a non-existent key is idempotent and returns 204; this function therefore returns Ok(()) in that case.