Add an API endpoint to trigger cache refresh

[soloradish]

Scenario:
We host a considerably large repository and have disabled the cache for immediate response. This has introduced a noticeable lag (1-3 seconds) when retrieving charts. If there were an API to trigger a cache refresh, we could enable caching with specified intervals. Then, after pushing a new chart, we could call this API to refresh the cache. This approach would allow immediate access to the new version and reduce the lag caused by a full scan.