Have only one instance of engine connection to Postgres

[kongzii]

Currently, we are using create_engine from sqlmodel on many places (just search for create_engine in PMAT and PMA), however, according to docs:

> You should normally have a single engine object for your whole application and re-use it everywhere.

The problem we started to see is that Postgres is erroring out, because too many connections are open.

I did a quick fix here, hopefully it will help, but this issue is still good to be implemented.