Add .env.example file for backend configuration (#2)

[Abdul0925]

Description

Resolves #2 - Adds a comprehensive .env.example file to the backend directory to provide clear guidance on required environment variables for local development setup.

Changes

• Created .env.example file with placeholder values for all required configurations
• Includes sections for server, database, and OpenAI
• Provides secure placeholder values instead of actual credentials

Environment Variables Included:

• Server configuration (PORT, NODE_ENV, CORS_ORIGIN, MAX_REQUEST_SIZE)
• Database connection (DATABASE_URL)
• OpenAI configuration (OPENAI_API_KEY, OPENAI_MODEL)

Impact

• Makes onboarding easier for new contributors
• Documents all required environment variables in one place
• Maintains security best practices
• Follows standard project configuration patterns

Testing

1. Copy .env.example to .env
2. Replace placeholder values with actual credentials
3. Verify application starts without environment-related errors