I use Ruby on Rails professionally and personally, and it addresses most of your features very easily (with Devise being a popular auth system that can be added).

Ruby follows duck-typing, so variables are not typed. If that was a strong point for you, Ruby might not be what you need.

I'm not quite sure about the admin UI (I usually use the rails console when I have things to change), though, but there's probably a gem that does exactly that (?).

Ruby on Rails is very powerful and intuitive, and I'll miss it if/when I change jobs and have to use some other language.

Edit:

> [...] so variables are not typed.

I might have used the wrong words... Ruby is dynamic, strongly typed. Variables are not limited to a certain type after they are created, and can change anytime.