Visual Paradigm is just great, having a taste of it for a year, but just expensive as well.
In a framework I created for a company using another backend language, I maintained the database via an erd. I generated crud modules also (with some extra configuration) - had no time to build an interpreter for erd + config iirc. The editor used for the backend language was eclipse, there was a great free plugin http://ermaster.sourceforge.net/content/manual/en/index.html that I used. I could generate the erd from an existing db and generate a dif (migration) from existing db and (maintained) erd.
In ruby someone built https://github.com/amatsuda/erd with comparable ideas. I have no idea if there is some js library for ERD modeling / rendering. The frontend would be a lot of work.
There is also an erlang lib for creating an erd from a postgres db https://github.com/Alexis-benoist/eralchemy
yAD works really well I hear nowadays.
for db diagrams dbdiagram is aweome and completely free