Support sort order for tags/list endpoint

Currently the `tags/list` endpoint returns tags in R2's default lexicographic key order. For registries with many tags spanning multiple versions, this means newer tags can end up on later pages, making it difficult to find the most recent images without paginating through all results.

It would be very useful if the endpoint supported a sort parameter (e.g., by upload time descending) so that the most recent tags appear first. This would make the tags/list endpoint much more practical for operational use — checking what's deployed, verifying a recent push landed, etc.

Thanks for maintaining this project!

Start by locating the tags/list endpoint implementation and reading how it currently paginates R2 keys. Determine whether upload timestamps are available for the returned tags, then define the sort parameter and verify that the newest tags appear first across pages.

Written by the indexing model from the issue text.