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/divzero_test.go

@@ -0,0 +1,43 @@
+package imgcache
+
+import (
+	"fmt"
+	"math"
+	"testing"
+)
+
+func TestSizePercentSafeWithZeroFetchBytes(t *testing.T) {
+	// Simulate the calculation from processAndStore
+	fetchBytes := int64(0)
+	outputSize := int64(100)
+
+	var sizePercent float64
+	if fetchBytes > 0 {
+		sizePercent = float64(outputSize) / float64(fetchBytes) * 100.0
+	}
+
+	// sizePercent should be 0, not Inf or NaN
+	if math.IsInf(sizePercent, 0) || math.IsNaN(sizePercent) {
+		t.Errorf("sizePercent = %f, should not be Inf/NaN", sizePercent)
+	}
+
+	// Should produce valid log output
+	result := fmt.Sprintf("%.1f%%", sizePercent)
+	if result != "0.0%" {
+		t.Errorf("formatted size ratio = %q, want %q", result, "0.0%")
+	}
+}
+
+func TestSizePercentNormalCase(t *testing.T) {
+	fetchBytes := int64(1000)
+	outputSize := int64(500)
+
+	var sizePercent float64
+	if fetchBytes > 0 {
+		sizePercent = float64(outputSize) / float64(fetchBytes) * 100.0
+	}
+
+	if math.Abs(sizePercent-50.0) > 0.001 {
+		t.Errorf("sizePercent = %f, want 50.0", sizePercent)
+	}
+}

internal/imgcache/service.go

@@ -153,8 +153,8 @@ func (s *Service) processFromSourceOrFetch(
 		}
 	}
 
-	// Fetch from upstream if we don't have source data
+	// Fetch from upstream if we don't have source data or it's empty
-	if sourceData == nil {
+	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,