feat: add batch search

[micpst]

Feature description

The idea is to allow the user to submit multiple questions in a single document search run. We should leverage batch requests to LLMs to mitigate timeouts/rate limits and ensure proper error handling.

Motivation

This feature is especially useful for evaluation when we process thousands of questions in different configurations, but also if we think about production workflows, batch processing is a must.

Additional context

No response

[mhordynski]

Connected with #346