Yes, the origin server always seeds videos uploaded on it through Webseed.
It is a BitTorrent extension that allow a server to seed a file through HTTP. It just need to serve statically a file, and then the clients will request chunks with a Content-Range HTTP header.
Not really. Reverse proxies like Nginx handle very well requests of static files. In my tests it can send chunks at 10MB/s without consuming more than 5% of CPU on a very small VPS.
No, 1000000 videos will represent around 2GB on PostgreSQL. It is acceptable for a video platform.
WEBM, MP4 or OGV videos.
If you already have followers, you can't.
If you don't: update your configuration and run NODE_ENV=production npm run update-host
to update the torrent files.