Enrich recipe generation endpoint

[jschoedl]

• add time estimates (prep time, cook time, oven time, total time - only total time required)
• add a short description for the preview
• a "generate more" button would be nice, which would call /generate again. for this, add a parameter existingRecipes or similar so that the AI knows the previous results and does not generate the same recipes again.
  • maybe title, portion count, nutrients and time estimate are enough (to keep the prompt short)
  • should work fine even if we input e.g. 25 existing recipes