fix: preflight neo4j driver before queries

fix: retry transient graph invocations

fix: recover neo4j graph connections

fix(api): degrade gracefully when graph is unavailable

fix(api): avoid asyncio shadowing in lifespan

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

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

test(4.3): add unit test suite — 31 tests, 0→full coverage on critical paths