Function precheck_base 

pub async fn precheck_base(
    pool: &PgPool,
    revision_id: Uuid,
    filename: &str,
    base: &BaseVersion,
    upload_sha256: Option<&str>,
) -> AppResult<()>

Checks a planned upload against the current file before its bytes are sent (chunked uploads announce their base first), so a conflict is reported before gigabytes are transferred. The binding check runs again when the version is stored.