Upload a sound file to the daemon's temporary sound directory.

The file is saved to /tmp/reachy_mini_sounds/<original_filename>. If a file with the same name already exists it is overwritten.

The upload is restricted to known audio extensions, capped at MAX_SOUND_UPLOAD_BYTES, and validated by content before being stored, so non-audio payloads cannot be written to disk.

Returns: JSON with the absolute path of the saved file on the daemon.