Add failing test for hot cache ContentType

Hot cache lookups must return ContentType to serve correct
Content-Type headers. Currently returns empty string.

internal/imgcache/cache_test.go

@@ -327,6 +327,60 @@ func TestCache_HotCache(t *testing.T) {
 	}
 }
 
+func TestCache_HotCache_ReturnsContentType(t *testing.T) {
+	cache, _ := setupTestCache(t)
+	ctx := context.Background()
+
+	req := &ImageRequest{
+		SourceHost: "cdn.example.com",
+		SourcePath: "/photos/hotct.jpg",
+		Size:       Size{Width: 800, Height: 600},
+		Format:     FormatWebP,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	// Store content
+	sourceContent := []byte("source data")
+	fetchResult := &FetchResult{
+		ContentType: "image/jpeg",
+		Headers:     map[string][]string{},
+	}
+
+	_, err := cache.StoreSource(ctx, req, bytes.NewReader(sourceContent), fetchResult)
+	if err != nil {
+		t.Fatalf("StoreSource() error = %v", err)
+	}
+
+	metaID, _ := cache.GetSourceMetadataID(ctx, req)
+
+	outputContent := []byte("output webp data")
+	_, err = cache.StoreOutput(ctx, req, metaID, bytes.NewReader(outputContent), "image/webp")
+	if err != nil {
+		t.Fatalf("StoreOutput() error = %v", err)
+	}
+
+	// First lookup populates hot cache
+	result1, err := cache.Lookup(ctx, req)
+	if err != nil {
+		t.Fatalf("Lookup() first error = %v", err)
+	}
+
+	if result1.ContentType != "image/webp" {
+		t.Errorf("Lookup() first ContentType = %q, want %q", result1.ContentType, "image/webp")
+	}
+
+	// Second lookup uses hot cache - must still have ContentType
+	result2, err := cache.Lookup(ctx, req)
+	if err != nil {
+		t.Fatalf("Lookup() second error = %v", err)
+	}
+
+	if result2.ContentType != "image/webp" {
+		t.Errorf("Lookup() hot cache ContentType = %q, want %q", result2.ContentType, "image/webp")
+	}
+}
+
 func TestCache_GetOutput(t *testing.T) {
 	cache, _ := setupTestCache(t)
 	ctx := context.Background()