feat(SESF-82): retrieval eval harness, freshness probe and search usage log - #101

Issues
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- Added 1
           

Complexity increasing per file
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- tools.py  11
- eval_retrieval.py  250
- search_log.py  13
- tests/test_eval_retrieval.py  77
- tests/test_ranking_settings.py  7
- tests/test_search_log.py  28
         

Complexity decreasing per file
==============================
+ rag_engine.py  -6