Production env config (.env.example + startup validation)

[krapfj23]

Summary

Create a .env.example template and add startup validation so the app fails fast with a clear error if required env vars are missing.

Blocked by

Nothing — this is a starting ticket.

Blocks

• Document local dev setup with Docker #55 (Docker dev docs)
• Deploy to cloud + configure env vars #60 (deploy to cloud)

Steps

1. Create backend/.env.example listing all required vars from cognee_config.py with placeholder values and comments
2. In app/cognee_config.py (or a new app/config.py), add startup validation: check that all required env vars are set
3. If any required var is missing, raise a clear error with the var name (fail fast, don't start the server)
4. Use pydantic-settings or simple os.getenv() with explicit checks

Acceptance Criteria

• .env.example lists all required vars with descriptions
• App fails to start with a clear error when a required var is missing
• App starts normally when all vars are present
• PR body includes Closes #57