fix: harden graph retrieval retries

feat(eval): RAGAS evaluation framework + RAG pipeline improvements

feat(retrieval): implement Reranker with score-gap filtering and FlashRank

feat(retrieval): implement VectorRetriever with rrf_merge and retrieve

feat: decouple frontend + multi-turn conversation + graph explorer + CAG layer

feat: Phase 0 complete — RERA RAG pipeline with FastAPI