feat: hybrid RAG pipeline upgrade

fix(rag-api): add msgpack and confluent-kafka to requirements, safe import

feat(rag-api): hybrid top stories — 3 from Kafka news.processed + 3 from DuckDuckGo

feat: add top stories API + fix reranker tokenizer

fix(news): Add time-based filtering and cache-control to /latest endpoint

feat: Add query enhancements and flexible prompting (v2.1)

fix: make _build_context and execute_chat async for hybrid search

feat(rag): implement hybrid search with live sources and production-grade intent classification