Add Cache Control

The current headers have zero cache control header which makes requests / responses a bit noisy with a lot of unnecessary re-downloads.

No file, test, or entry point is named in the issue. Start by locating where request and response headers are assembled in the repository, then determine the intended Cache-Control behavior and verify that requests no longer trigger unnecessary downloads. Add focused coverage if an existing header test is found.

Written by the indexing model from the issue text.