Not sure if it quite fits the bill, but if your end user video uploads are stored on S3 or similar then zencoder does most of what you are looking for.
I came across this product recently: https://www.crowdsupply.com/gnubee/personal-cloud-1 which is seeking crowd funding at the moment. Personally I think they've made a mistake by going for 2.5" storage first with 3.5" coming later. Anyway, it sounds like it might be of interest to you?
This particular setup can/will leak cookies set on foo.tld to ads.foo.tld which leads to security concerns - only in your specific example though. If you were to use www.foo.tld and ads.foo.tld then you're good to go. While it is possible to carefully setup cookies that do not leak, its something you need to be careful of when recommending. That said, I do agree with your premise and wish native ad hosting like this happened much more often.
For anyone familiar with the principles of personal finance and investing, my trick was unimpressive and easily reproducible: Just spend much less than you earn and pour the difference into efficient index funds.
This is the usual US centric nonsense where a certain privileged few can use their privilege to get out of the rat race, whereas the vast majority of the planet can't even get into that race in the first place, never mind hack it for these sort of benefits. I'm rather sick of the bullshit claims that there is no magic and that anybody else can do this too - this is patently untrue. You are ultra privileged; you lucked out in the ethnic and circumstances lottery. By all means share how you did it, but please stop intimating thats its at all possible for the vast majority of the rest of us to do this.