[FEAT]: add PostgreSQL storage backend

[siddharth3690]

Summary

The README mentions that the storage backend currently uses SQLite and that PostgreSQL support is "coming soon". I couldn't find an open issue tracking this work, so opening one to discuss scope and subtasks.

Motivation

• PostgreSQL is more suitable for production deployments than SQLite in many environments.
• Having a clear tracking issue can help contributors (like me) pick up well-scoped pieces of the work.

Initial thoughts / possible subtasks

• Define configuration flags for selecting the DB backend (SQLite vs PostgreSQL).
• Model schema and migrations for PostgreSQL.
• Abstract storage layer so both SQLite and PostgreSQL implementations share a common interface.
• Add basic tests that run against PostgreSQL (maybe via Docker in CI).
• Update documentation to show how to configure and run CtrlB Control Plane with PostgreSQL.

Questions

• Do you already have an internal design for the PostgreSQL backend?
• Would you be open to community contributions on smaller parts ?

I'm a student contributor and I might not be able to implement the entire feature, but I'd like to help with some pieces if possible.