feat: 实现 BM25 稀疏 + 稠密向量混合检索功能

backend/rag_core/__init__.py

@@ -6,6 +6,7 @@ RAG Core - 公共 RAG 组件包
 
 from .embedders import LlamaCppEmbedder
 from .vector_store import QdrantVectorStore
+from .sparse_embedder import BM25SparseEmbedder, get_sparse_embedder
 from .store import PostgresDocStore, create_docstore
 from .retriever_factory import create_parent_retriever
 from .config import (
@@ -21,6 +22,8 @@ from .config import (
 __all__ = [
     "LlamaCppEmbedder",
     "QdrantVectorStore",
+    "BM25SparseEmbedder",
+    "get_sparse_embedder",
     "QDRANT_URL",
     "QDRANT_API_KEY",
     "LLAMACPP_EMBEDDING_URL",