Dump image to disk when we see <20 pucks

When we see less than 20 pucks we want to get some history of why so that we can fix it going forward.

## Acceptance Criteria
* When the `/result` endpoint is called and we find <20 images we dump the image to disk somewhere along with data about which pucks we did see
* We only ever keep a small set of these images (e.g. ~20) when there are more we overwrite previous ones

Locate the handler for the `/result` endpoint and trace how detected puck data and the image are available there. Decide where the diagnostic images and puck data should be stored, then verify that fewer than 20 pucks triggers capture and that retaining roughly 20 images overwrites older entries.

Written by the indexing model from the issue text.