Revise update of scraper targets

Currently the patch/update process for scraper targets does not use any struct to specify which fields can be updated, but uses the `platform.ScraperTarget` directly.

Thus I propose to introduce a `platform.ScraperTargetUpdate` struct and use another private struct to decode into it during `PATCH`.

Start by locating the PATCH handler for scraper targets and the definition of platform.ScraperTarget. Review how the request body is decoded and which fields are currently accepted. Done means introducing the proposed update struct and private decode struct so PATCH updates only the intended fields.

Written by the indexing model from the issue text.