Classify an existing food by rxfood_id (internal testing)

Phase 1 admin-only testing endpoint for the LLM food classifier. Runs the 8-step pipeline on the food identified by rxfood_id and returns the predictions, per-step LLM metadata, deterministic overrides, disagreements with the current DB values, and total cost. Writes predictions to the DB (food / food_cc / food_ranking + food_data_source provenance) only when commit=true. Do NOT invoke from production paths.