Add detailed logging for image requests with cache status and timing

internal/imgcache/imgcache.go

@@ -90,6 +90,8 @@ type ImageResponse struct {
 	LastModified time.Time
 	// CacheStatus indicates HIT, MISS, or STALE
 	CacheStatus CacheStatus
+	// FetchedBytes is the number of bytes fetched from upstream (0 if cache hit)
+	FetchedBytes int64
 }
 
 // CacheStatus indicates whether the response was served from cache.

internal/imgcache/service.go

@@ -172,6 +172,7 @@ func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*Imag
 		Content:       io.NopCloser(bytes.NewReader(processedData)),
 		ContentLength: int64(len(processedData)),
 		ContentType:   processResult.ContentType,
+		FetchedBytes:  int64(len(sourceData)),
 	}, nil
 }