Is there any database modeling software which provides a real-time (and editable) ERD, which I can manipulate alongside a traditional GUI?
The two worlds differ because of "state". A web page has none; a database is nothing but.

A web page is rendered and it is over with. Yes, a user can click here and there, type values and widgets may appear and go again, but nothing permanent has changed as far as the enterprise is concerned. Should the rendering computer fail no inconsistency can be introduced. The page can be completely re-created as it was with a single call to the server. A change to, say, some CSS does not have to account for every other piece of styling that has been applied to this page throughout history.

A database, however, must maintain and protect state. If the DB server fails it must (should!) guarantee the integrity of the data it has acknowleded as successfully written. Rebuilding the current data from all past input would be time-consuming, assuming the previous files (key strokes?) exist and can be processed in the identical sequence. A change to a table's schema must take account of existing data's structure and meaning. Declared referential integrity (DRI) constraints, which further guarantee's data quality, must be honoured.

NoSQL tries to side step this issue by allowing dynamic schema i.e. no to rows need have the same columns, data types or meaning. This allows very rapid evolution of the applicaiton without hinderance from database upgrade tasks. The responsibility for ensuring the completeness, correctness and consistency of the data is now in the hands of the application developer with little or no help from the persistence tool. Being able to Deal with all the existing "state" in the previous format(s) is now her responsibility, too.

The difference between logical and physical models is a valid one, too. In my experience, however, they tend to be very similar. The mapping from logical to physical must be explicit within the modelling tools (if it is to generate DDL) so could be made transpartent to code written against the logical model. I don't think this is a great impediment to the world you hope for.

I have used CASE tools where the logical data model can be entered into the tool, procedural logic written against that model immediately, and a functioning system generated at the touch of a button. It was a great tool to work with. It was, however, expensive and seems to have declined.

There are tools which will take a "desired state" model, compare it to an "existing state" database and generate the delta DDL. This can be integrated into a continuous build &amp; deploy workflow. Having seen a generated change script which was so awful it would take in excess of three days to complete, however, I'd recommend reviewing manually before submitting.