Added django-environ for environment-based settings configuration

[arman-sharker]

Summary

This PR integrates django-environ to manage sensitive Django settings like database credentials via environment variables.

What it does

• Uses django-environ to manage settings via environment variables.
• Reads environment variables from .env.example.
• Updates DB Config to read credentials via env().

Why

• Follows best practices for config management (12-Factor App).
• Separates code from configuration.
• Improves security and flexibility across environments (dev/staging/prod).

Dependency Update

• Added django-environ to requirements/common.txt