Function store_user_private_key 

pub async fn store_user_private_key(
    dsn: &str,
    private_pem_b64: &str,
) -> Result<(), DBError>

Stores a user’s RSA private signing key into their (already-provisioned) per-user database at dsn. The private key never leaves that database.

Errors

DBError::Sqlx on connect or insert failure.