Move torch and sentence-transformers to requirements-reranker.txt so
the main Docker build succeeds even if these large packages fail to
install. The reranker code already handles missing imports gracefully
when RERANK_ENABLED=false (the default).
This fixes the production deployment — builds were failing because of
the ~800MB torch dependency, preventing ALL new code from deploying.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>