harperdb register throws crypto error on bad license key

Running `harperdb register` with an invalid license key throws a cryptography error instead of a user-friendly validation message.

🤖 Filed by Claude on behalf of Kris.

Start by reproducing the cryptography error from the `harperdb register` command with an invalid license key, then trace the registration and license-validation entry points. The work is done when invalid keys produce a user-friendly validation message instead of the crypto error; add or update the relevant test if the existing test structure identifies one.

Written by the indexing model from the issue text.