docs: Add .env.example with all required environment variables

[Noel24Dav]

The README references env.example in the setup steps with cp env.example .env, and even documents the required environment variables in the Configuration section — but the actual env.example file is not confirmed to exist in the repository. If it's missing, the setup instructions break at step one.
What's missing:

A committed env.example file with all required variables pre-filled with safe placeholder values
Variables for Django, PostgreSQL, Redis, JWT, Celery, and any third-party integrations are documented in the README but not consolidated into a ready-to-copy template