Document PostgreSQL requirement for full test suite when using SQLite local setup

Hi,

I set up VulnerableCode locally in GitHub Codespaces using the documented Makefile flow.

What worked:
- make dev
- make envfile
- make sqlite
- make run

The application started successfully with SQLite for local development.

What I observed:
- make test fails under SQLite because some tests rely on PostgreSQL-specific database features, including DISTINCT ON queries and JSON containment lookups.

The installation docs already mention SQLite caveats, but it would help contributors if the docs clearly stated that:
- SQLite is fine for quick local development and running the dev server
- PostgreSQL should be used for running the complete test suite

I’ve prepared a small docs clarification for this.