Fix SSE route dependency and align architecture docs

backend/app/infrastructure/vectorstore/dense_retriever.py

@@ -0,0 +1,24 @@
+"""Implement infrastructure support for dense retriever."""
+
+from __future__ import annotations
+
+from app.domain.retrieval import EmbeddingProvider, RetrievalQuery, Retriever, RetrievedChunk, VectorIndex
+# Keep adapter behavior explicit so integration details remain easy to audit.
+
+
+
+class DenseRetriever(Retriever):
+    """Provide the Dense Retriever retriever."""
+    def __init__(self, *, embedding_provider: EmbeddingProvider, vector_index: VectorIndex) -> None:
+        """Initialize the Dense Retriever instance."""
+        self.embedding_provider = embedding_provider
+        self.vector_index = vector_index
+
+    def retrieve(self, query: RetrievalQuery) -> list[RetrievedChunk]:
+        """Handle retrieve for the Dense Retriever instance."""
+        query_vector = self.embedding_provider.embed_query(query.query)
+        return self.vector_index.search(query_vector, query.top_k, query.filters)
+
+    def search(self, query: str, top_k: int, filters: str | None = None) -> list[RetrievedChunk]:
+        """Handle search for the Dense Retriever instance."""
+        return self.retrieve(RetrievalQuery(query=query, top_k=top_k, filters=filters))