Categorize Airtable invoices with OpenAI and TOON token optimization

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> ⚠️ Notice: > This workflow uses the CustomJS JSON to TOON node from CustomJS, which requires a self-hosted n8n instance and a CustomJS API key. This workflow demonstrates how to automatically categorize invoices from Airtable using OpenAI, while significantly reducing token usage by converting structured data into TOON (Token-Optimized Object Notation) before sending it to the LLM. Instead of passing large, nested JSON payloads (clients, invoices, invoice items) directly to OpenAI,