fix: guard against division by zero when fetchBytes is 0

processAndStore() computed sizePercent as outputSize/fetchBytes*100
without checking for zero, producing Inf/NaN in logs and metrics.

Also treat empty cached source data the same as missing (re-fetch
from upstream) since zero-byte images can't be processed.

Fixes #5

internal/imgcache/service.go

@@ -153,8 +153,8 @@ func (s *Service) processFromSourceOrFetch(
 		}
 	}
 
-	// Fetch from upstream if we don't have source data
-	if sourceData == nil {
+	// Fetch from upstream if we don't have source data or it's empty
+	if len(sourceData) == 0 {
 		resp, err := s.fetchAndProcess(ctx, req, cacheKey)
 		if err != nil {
 			return nil, err
@@ -264,7 +264,11 @@ func (s *Service) processAndStore(
 
 	// Log conversion details
 	outputSize := int64(len(processedData))
-	sizePercent := float64(outputSize) / float64(fetchBytes) * 100.0 //nolint:mnd // percentage calculation
+
+	var sizePercent float64
+	if fetchBytes > 0 {
+		sizePercent = float64(outputSize) / float64(fetchBytes) * 100.0 //nolint:mnd // percentage calculation
+	}
 
 	s.log.Info("image converted",
 		"host", req.SourceHost,