Add external API bindings for meat pie on demand

One pie is never enough - we should add a binding to an external API that can provide us with more pie.

We should look for candidate APIs that support a free tier of reasonable meat pie search.
We should integrate this API for pies on demand (i.e. download an image from the search results and return it).

We should decide whether this on-demand pie should be cached or ephemeral.

No files, tests, or entry points are named in the issue, so first survey the repository to find how pie requests are handled. Compare external APIs with a free tier and define whether results are cached or ephemeral. Done means the chosen API is integrated, a meat pie image can be downloaded and returned on demand, and the caching behavior is documented.

Written by the indexing model from the issue text.