docs: clarify PostgreSQL requirement for full test suite

[Sanjay-VK07]

Summary

Clarify that SQLite works for quick local development, but the complete test suite requires PostgreSQL-specific database features.

What I observed

While setting up VulnerableCode locally in Codespaces:

• make sqlite worked for local setup
• make run started the development server successfully
• make test failed under SQLite because some tests rely on PostgreSQL-specific features such as DISTINCT ON queries and JSON containment lookups

Change in this PR

This updates the installation docs to clarify that:

• SQLite is suitable for quick local development
• PostgreSQL should be used when running the full test suite

Related to #2229

[Sanjay-VK07]

Hi maintainers, this PR documents an issue I ran into while setting up VulnerableCode in Codespaces with SQLite for local development.

The app ran successfully with make sqlite and make run, but make test failed because some tests rely on PostgreSQL-specific features. I added a small clarification in the installation docs so new contributors know SQLite is fine for quick local setup, while PostgreSQL is needed for the full test suite.

Happy to update the wording if you’d prefer a different phrasing. Thanks.