Add monitoring and stats for the raster server

It would be good to know how many requests are hitting the server, maybe get errors in Sentry, and have a frequent test that hits the server and ensures it's still responding with valid PNGs.

Start by locating the raster server entry point and the existing test setup. Determine how request counts and errors could be observed, then add a frequent check that confirms the server responds with valid PNGs; done means monitoring is available and the check reliably detects invalid or unavailable responses.

Written by the indexing model from the issue text.