feat(rag): 混合检索融合 — Milvus 向量 + Bleve 全文 + RRF + DashScope rerank

检索从向量单路升级为混合：向量(Milvus) + 全文(Bleve BM25) → RRF 融合 →
可选 rerank(DashScope gte-rerank)。

- rag/bleve.go: Bleve 全文索引(内存,随 ingest 写入;kb 过滤)；ingest 同步写 Milvus+Bleve
- rag/fuse.go: RRF(Reciprocal Rank Fusion, k=60, 按文本去重)融合多路排序
- rag/rerank.go: DashScope gte-rerank 客户端(可选,env 配置,失败降级 RRF)
- rag/rag.go: Search 改混合(向量+全文→RRF→可选rerank→topK)；main 读 RERANK_* env
- 验证: 全模块 build✓ + e2e PASS; live——入库写双索引；查'NATS'→全文精确命中#1+向量
  →RRF NATS 排首(向量=4 全文=1)；接 DashScope gte-rerank(百炼 key 有权限)→relevance
  score 0.19 真重排；retriever 节点端到端→DeepSeek 答 Milvus
- 边界: Neo4j 图路(GraphRAG,需实体抽取)推迟；Bleve 内存索引重启重建；rerank 走 env
  (TODO 同 embedding 搬控制面 kind=rerank)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go.work.sum

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