serve_image and serve_output_image now reject anything whose extension
is not in IMAGE_EXTENSIONS, fail closed when no output directory is
configured, and answer 403 (not 404) for dot-dot, absolute and
symlink-escaping paths. Containment is checked after realpath+normcase
so it behaves the same on Windows, macOS and Linux. 500 responses use
safe_error_message so OSError text no longer leaks absolute paths.