Add Config Module with Environment Validation

[Mkalbani]

Description

Set up a centralised config module using NestJS ConfigModule that validates all required environment variables at startup. Missing or malformed config should cause the application to fail fast with a clear error — not silently misbehave in production. All other modules should consume config through this single module rather than reading process.env directly.

Acceptance Criteria

• A config module is scaffolded at src/config/
• @nestjs/config with Joi validation is used for env schema enforcement
• Required variables include: DATABASE_URL, JWT_SECRET, JWT_EXPIRY, PORT
• Application refuses to start if required env vars are missing or invalid
• Config values are injectable via ConfigService across all modules
• Separate .env.example documents all required and optional variables
• Unit tests verify that missing required vars throw a startup error