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

[Sanjay-VK07]

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.

[Sanjay-VK07]

Hi, I’ve opened a PR with a docs clarification for this: #2230