Case sensitivity

It looks like contribcard currently considers the case of a GitHub handle. This causes problems when integrated with CertDirectory user profiles.

For example:

✅ Works: [User 1 profile](https://certdirectory.io/profile/e9465932-175e-468f-9d4d-61f104db9c01)

❌ Broken: [User 2 profile](https://certdirectory.io/profile/af109dac-e461-45b7-add9-db75dea99be6)

Other integrations (like DevStats score and GitHub heatmap) work fine for the second user, since GitHub itself ignores case.

**Proposed fix**
Update contribcard to handle GitHub usernames in a case-insensitive manner.
i.e
Both of the following calls should return the card
https://contribcard.cncf.io/galphaa
https://contribcard.cncf.io/Galphaa

Start by tracing how contribcard handles the username in the two URLs given in the issue and how it retrieves the corresponding GitHub profile. Add or update coverage for both lowercase and mixed-case requests, then verify that both return the card for the same user.

Written by the indexing model from the issue text.