fix(H-06): add lockless pre-check in get_driver() -- prevents lock contention serializing all requests under load

fix(BUG-20): add 30s TTL cache for Neo4j connectivity check -- prevents connection storm on cold start

fix(BUG-01): reconnect stale Neo4j driver — was causing intermittent HTTP 500s

fix(api): graceful startup without Neo4j + HEAD support on /health

feat(api): FastAPI backend with entity search, dossier, graph, and risk endpoints