This is not "the" code but here's a script. I can't take credit for it... I don't remember but I probably cobbled it together by taking stack overflow code and making it more friendly. First, you need ffmpeg (ymmv.. this assumes macOS, for others installing ffmpeg is also possible but left as an exercise for the reader):

    $ brew install ffmpeg

[edit: fixed some typos]

    #!/bin/bash

    export OLD_IFS=$IFS
    export IFS=''

    mkdir -p converted
    for a in *.{webm,mkv,ts} 
    do
        ffmpeg -i "$a" -c:v libx264 -preset slow -crf 20 -c:a libvo_aacenc -b:a 128k "$a.mp4"
        if [[ $? == 0 ]]; then
            mv "$a.mp4" converted
        fi
    done
    export IFS=$OLD_IFS

Not saying this is perfect. Take it as a proof of concept. And there's no web UI, nor would I want one. Suggestions for tweaks welcome.

> $ brew install ffmpeg

FWIW, there's a `brew` fork for linux: https://github.com/Linuxbrew/brew

It's not especially useful in this case as ffmpeg is available in linux package managers, but it's handy for tools that aren't.