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.