fix: add lightgbm and xgboost to requirements.txt

Missing packages caused XAI service to fail on startup with
"No module named 'lightgbm'" error in HF Spaces container.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -33,6 +33,8 @@ numpy>=1.26.3
 pandas>=2.1.4
 scipy>=1.11.4
 scikit-learn>=1.3.2
+lightgbm>=4.3.0
+xgboost>=2.0.3
 
 # === API & Web ===
 aiohttp>=3.9.1