Function mark_failed 

pub(crate) async fn mark_failed(
    pool: &PgPool,
    id: i64,
    error: &str,
) -> Result<(), Error>

Marks a stream as permanently failed.

Sets upload_status = 'failed' and records the error message. Called when the upload processor decides not to retry (e.g., max attempts exceeded or a non-retryable HTTP status).

Errors

Returns sqlx::Error on query failure.