feat: add Docker Registry V2 endpoint at /v2/ (#57)

The v3 Go rewrite removed the /v2/ Docker Registry compatibility endpoint. Docker clients need:
- GET/HEAD /v2/ → 200 (registry ping)
- GET/HEAD /v2/{remoteName}/* → proxy to the docker remote

Usage: `docker pull artifactapi.example.com/{remoteName}/image:tag`
Reviewed-on: #57
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/server/server.go

@@ -96,6 +96,7 @@ func (s *Server) routes() chi.Router {
 
 	proxyHandler := v1.NewProxyHandler(s.engine, s.virtEngine, s.db, s.store, s.localHandler)
 	r.Mount("/api/v1", proxyHandler.Routes())
+	r.Mount("/v2", proxyHandler.DockerV2Routes())
 
 	remotesHandler := v2.NewRemotesHandler(s.db)
 	virtualsHandler := v2.NewVirtualsHandler(s.db)