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/service.go

@@ -103,6 +103,7 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 			"host", req.SourceHost,
 			"path", req.SourcePath,
 		)
+
 		return nil, fmt.Errorf("%w: %w", ErrUpstreamError, ErrNegativeCached)
 	}