Function mint 

pub fn mint(
    conn: &Connection,
    pepper: &[u8; 32],
    mode: Normalise,
    meta: &BatchMeta<'_>,
    max_attempts: u32,
    generate: impl FnMut() -> String,
) -> Result<Option<String>, String>

Generate-check-register in one call, retrying up to max_attempts times on collision. generate is supplied by the caller (unv-server), which owns the id-shape decision this module has no opinion about.