CVE-2026-21720 Unauthenticated DoS: avatar cache leaks goroutines when /avatar/:hash requests time out

Every uncached /avatar/:hash request spawns a goroutine that refreshes the Gravatar image. If the refresh sits in the 10-slot worker queue longer than three seconds, the handler times out and stops listening for the result, so that goroutine blocks forever trying to send on an unbuffered channel...