Fix AI diet callback contract

[cfcromn]

Summary

• update AI save flow to create slots through /diets/ai-callback with X-Internal-Token
• keep /meals as the menu/nutrition fill step using the returned slot id
• pass INTERNAL_API_KEY in prod compose and update the mock backend/test

Root cause

Hololog-Server changed slot creation from POST /diets to POST /diets/ai-callback and now requires X-Internal-Token. Hololog-AI was still using the old callback contract.

Validation

• uv run python -m compileall app test
• uv run python -m unittest test.test_ai_callback_contract

Closes #8