Set up OpenAI API key for Postiz AI agent

## Steps 1. Create an OpenAI API key at `platform.openai.com/api-keys` 2. Set a spending limit ($5-10/mo) 3. Add `OPENAI_API_KEY` env var in Railway → otf-postiz → Postiz service 4. Redeploy 5. Test the Agent feature in Postiz sidebar