Add per-host connection limits for upstream fetching

- Add upstream_connections_per_host config option (default: 20)
- Implement per-host semaphores to limit concurrent connections
- Semaphore released when response body is closed
- Prevents overwhelming origin servers with parallel requests

internal/handlers/handlers.go

@@ -72,6 +72,9 @@ func (s *Handlers) initImageService() error {
 	// Create the fetcher config
 	fetcherCfg := imgcache.DefaultFetcherConfig()
 	fetcherCfg.AllowHTTP = s.config.AllowHTTP
+	if s.config.UpstreamConnectionsPerHost > 0 {
+		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
+	}
 
 	// Create the service
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{