Default database backend for installer

[SchrodingersGat]

For the docker setup, as well as the devcontainer, we specify postgres as the default database backend. This has proven very solid - no issues really.

However the linux installer setup defaults to sqlite - which is fine for small setups but can run into significant issues as the database grows / more users / big queries.

References

• [db] Backend setting improvements #11500 - some changes to the default sqlite connection options to reduce concurrency locking issues
• Unable to consume items #11493 - likely the same issue

We should consider moving the installer to use postgres as the default - otherwise we are locking in users to a sub-standard default, and it's much harder to change backend later on.

@matmair thoughts on this one? I think we are setting users up for failure by using sqlite "out of the box"

[matmair]

that means careful handling of database install and setup (what happens if postgres is already installed - do we create a inventree database with the root access we have? for example). I'd rather just add a system warning or doc line

[matmair]

for context: the postgres setup and maintenance tasks are the only thing that keeps breaking on the digital ocean marketplace image again and again. It always consumes a few hours to fix and makes testing much more of a pain