redesign paths in video class

[kj-9]

Currently, the video class path is tied to an absolute path:
Example

    video_path":"/Users/kh03/work/repos/nakaji-map/data/videos/-1Rvhiu0Giw/video.mp4",
    frames_dir":"/Users/kh03/work/repos/nakaji-map/data/videos/1Rvhiu0Giw/frame",
    frames":
        {
            path":"/Users/kh03/work/repos/video-ocr/data/videos/1Rvhiu0Giw/frame/frame-0.png",

video_path, frames_dir, frames.path is an absolute path prefixed by data_dir,
user names like data_dir are privacy sensitive.
They are also not portable, since they embed data_dir, which must be set by a variable.