Fix logging: add response_bytes to middleware, cache_key to handler

- Middleware now tracks and logs bytes written via response_bytes
- Handler logs cache_key for cache hit debugging
- Changed "served encrypted image" to "image served" (only URL is encrypted)

internal/handlers/imageenc.go

@@ -91,7 +91,8 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 
 		// Log completion
 		duration := time.Since(start)
-		s.log.Info("served encrypted image",
+		s.log.Info("image served",
+			"cache_key", imgcache.CacheKey(req),
 			"host", req.SourceHost,
 			"path", req.SourcePath,
 			"format", req.Format,