Artifacts had no size limit. A fetch was bounded only by the server's write
timeout, and storage.max_size evicts only after the bytes are written, so
any client could fill storage or keep fetches busy by requesting huge
artifacts.
- Add artifact_max_size (PROXY_ARTIFACT_MAX_SIZE), unlimited by default
- Refuse an artifact whose declared size is over the limit before reading
it, and fail one of unknown size once the download passes the limit
- Apply it in the fetcher so stored, streamed and mirrored artifacts all
honour it; nothing is cached and clients get 502