Function mint 

pub async fn mint(
    user_id: Uuid,
    ttl: i64,
    token_type: TokenType,
    generation: i32,
) -> Result<TokenDetails, MintError>

Mints a token of token_type for user_id, signed with that user’s private key. generation is threaded in by the caller rather than re-read here: login already holds it from the password check and refresh from the verified token, so minting costs no extra query.