fix: resolve all golangci-lint errors

- Add blank lines before return statements (nlreturn)
- Remove unused metaCacheMu field and sync import (unused)
- Rename unused groups parameter to _ (revive)
- Use StorageFilePerm constant instead of magic 0600 (mnd, gosec)
- Add nolint directive for vipsOnce global (gochecknoglobals)

internal/imgcache/storage.go

@@ -16,6 +16,8 @@ import (
 const (
 	// StorageDirPerm is the permission mode for storage directories.
 	StorageDirPerm = 0750
+	// StorageFilePerm is the permission mode for storage files.
+	StorageFilePerm = 0600
 	// MinHashLength is the minimum hash length for path splitting.
 	MinHashLength = 4
 )
@@ -409,7 +411,7 @@ func (s *VariantStorage) Store(key VariantKey, r io.Reader, contentType string)
 		return 0, fmt.Errorf("failed to marshal metadata: %w", err)
 	}
 
-	if err := os.WriteFile(metaPath, metaData, 0640); err != nil {
+	if err := os.WriteFile(metaPath, metaData, StorageFilePerm); err != nil {
 		// Non-fatal, content is stored
 		_ = err
 	}